Project Rear Disc Brakes

[SmithvilleD]

If needed, that is the type of proportioner I've used before. But I need to see if adjustment is needed, & then better understand how the stock proportioner is fit (incorporated into antilock?).

As background on what brought me to this point, my truck's rearend is worn & needed new bearings/ring/pinion. About that time, I found an '04 K2500 rolling chassis on Craigslist for $350-400 (can't recall now). Bought it for the rear diff but couldn't pass up scrounging more potentially useful parts from it.

I'd recently completed rebuilding my 26' GN Superwide Carhauler, which weighs 5500# empty. Truck had fresh good quality friction on the stock discs/drums & trailer had new brakes.

The second pic is of truck & a trailer load of landscaping block making total truck/trailer weight around 18.5k lbs. With this load, and rolling low & slow into my home town at around 35 mph, someone accidentally pulled out in front of me. Granted the truck wasn't designed to haul or stop this load. And 33" tires make it worse. But I was still disappointed that it was all I could do to get the truck's brakes close to locking - at this low speed (trailer brakes performed as they should & were set so they locked just after I got one of the fronts close to locking.)


Fortunately I was going just barely slow enough to avoid the collision. My point is I know I'm beyond what the truck was designed to do. But it's still gonna occasionally be called upon to do it. I enjoy this type of work & had the parts I think should prove to do the job better. They certainly do on the GMT800 3/4 & 1T's.

 

[SmithvilleD]

Posted wrong pic before. This is an example of the rolling setup at around 18,500 lbs.

 

[Will L.]

On concearn of understeer/oversteer being an exact science in a truck/suburban. Um...no.

Compare a 160 lbs driver alone, vs 7 passengers at 220 lbs each get in. Hooked up to the camp trailer.

Entirely different results.

I've always been into drag racing. I have a cousin that ran a bit in nascar. My dad had a 34 -5 window that saw its share of the salt flats in the day. Other than high power to weight ratio what did you have to tune? Brakes. It's not a huge mystery, there is no exacting science to it. Make improvements, work out the kinks, enjoy the ride.

Have you ever changed out factory front pads for better stopping ones? Did you do the same ratio of stopping power to match and do your tears at the same time? No. You just put on new pads, maybe a couple sets before finally doing the rears. It's not that huge of a panic factor.

The gmt400 get fixed into the gmt800? No. My 2001 suburban 2500 got front brakes around 55,000 miles. The rears at 200,000. Hmm.

I've seen the 1 load BigT hauled up to Big Bear when we met. Hauling much of anything back down those roads, he NEEDS major braking improvement.

Dude, make the improvements, your on the right tack with an adjustable valve. Thousands of rigs have had brake improvements made, you can too. Notice Brembos don't come with a new valve, even in a kit.

 

[Big T]

On concearn of understeer/oversteer being an exact science in a truck/suburban. Um...no.

Compare a 160 lbs driver alone, vs 7 passengers at 220 lbs each get in. Hooked up to the camp trailer.

Entirely different results.

I've always been into drag racing. I have a cousin that ran a bit in nascar. My dad had a 34 -5 window that saw its share of the salt flats in the day. Other than high power to weight ratio what did you have to tune? Brakes. It's not a huge mystery, there is no exacting science to it. Make improvements, work out the kinks, enjoy the ride.

Have you ever changed out factory front pads for better stopping ones? Did you do the same ratio of stopping power to match and do your tears at the same time? No. You just put on new pads, maybe a couple sets before finally doing the rears. It's not that huge of a panic factor.

The gmt400 get fixed into the gmt800? No. My 2001 suburban 2500 got front brakes around 55,000 miles. The rears at 200,000. Hmm.

I've seen the 1 load BigT hauled up to Big Bear when we met. Hauling much of anything back down those roads, he NEEDS major braking improvement.

Dude, make the improvements, your on the right tack with an adjustable valve. Thousands of rigs have had brake improvements made, you can too. Notice Brembos don't come with a new valve, even in a kit.

Will as I said, we're doing this in increments. I've already established the lower rear pressure for drum set ups, so my ass is not coming around to kiss me in a panic stop. The real issue to focus on is what Ferm has pointed out and that's the resultant brake pedal height. I'm hoping it's not much of an issue on the front brake conversion. if it is, then I need to install the GMT 800 master cylinder and hydro boost. Again, incremental adjustments.

I'll report back. We'll be reaming knuckles the 2nd week of April.

 

[Big T]

Why do your pads on your Suburbans last only 15K? So many factors. Type/manufacturer of pads, driving style, mechanical issues (like caliper(s) hanging up/collapsed brake hose/piston(s) hanging up in the caliper bore), maintenance or lack thereof, who knows, but that's really poor service life.

My 98 K2500 Suburban has 38K on its set of front pads and they're 3/16" from the wear indicators. I'm running ceramic pads with drilled rotors up front with braided SS hoses and kevlar rear drum shoe linings. My driving is about 75-80% stop-and-go city and the rest highway. Every time I change pads, I also replace ALL wear items such as caliper slider bolts, boots, etc. and properly lubricate all moving/wear points with the correct lube. I have had no premature wear problems with the front pads on either the Burb, or my 94 C2500 C&C work truck.

The 94 work truck, with its service box full of tools and the ladders and siding brake on it, weighed in at 7900# "empty" and with a house full of replacement windows, entry doors and a sliding patio door (23 windows, three doors, three storm doors, a patio, four boxes of trim coil, lumber and a roll of insulation on one farm house job I did) the truck scaled at 10,280# loaded (why I plated it at 6 tons). I ran severe duty pads with drilled rotors and SS braided lines up front and kevlar linings on the rear. My driving was about 75/25 Hwy/City as I covered 2/3 of Nebraska, 1/3 of Iowa and parts of Missouri, Kansas and South Dakota. I still got 30K out of the front pads. I had to do the front brakes about twice a year since I was averaging 70-75K a year.

Oh, and I've never had any problems going "whoa" with either of them, even the 94 when it was loaded to the gills or pulling a rental trailer with surge brakes and a 700 series Bobcat on it, either.

Two reasons:

1) I regularly make a trip up to Big Bear Lake ever the base elevation is 1,500' and the top elevation is 7,100' or 8,500', depending on the route.

2) I regularly tow a 27' boat. Trailer has electric over hydraulic disc brakes, but it it's still hard on the brakes.

I'm using the best Raybestos pads available, recommended here by non other than Fermanator. Brand new reman calipers and new Raybestor Advaced Technology Slotted Rotors. Nothing but the best and they were done in less than 15K miles. Same thing for my son's.

I bought my '99 near Andover, which is south of Moline,IL. Drove through Lincoln, NE as I headed to California. Sorry, but you's got awfully flat country out there compared to what we've got.

 

[Big T]

And here's some pics of my Conch 27 with Volvo KAD 44 diesel (260 hp from 3.6 liters):

 

[THEFERMANATOR]

I've towed some big loads with mine, and I tend to be a bit more aggressive with my brakes than most as I can snap the speedo up faster than most(when you snap one up, you have to be able to reel it back down). My pads always seem to last over 30K miles, but I'm normally changing them out because of warped rotors more than anything else. I still don't understand why people complain about the brakes in these things. I have the 3500 DRW stuff on it now(larger calipers, thicker rotors, and larger wheel cylinders) with stainless braided brake lines, and I have NO problems throwing somebody out of the seat and choking them with the seat belt. After I did the better pads up front before, I did have to much front brake bias, so I put in the 1 3/16" wheel cylinders, then I had to much rear brake bias, then I finished it off to all DRW stuff, and now that it's got some miles on them, I'm happy as can be. Pad life on the GMT-400 is going to suffer for one simple reason, the pads are dinky. You have a MASSIVE single piston caliper which means you can't run a long pad because of the single piston. Going to dual pistons allows for a MUCH longer pad to distribute the braking force out across, hence why theres so many people with the newer trucks going 100K miles or more on front pads. I know people that got 200K out of the rears, and did the fronts just because they were doing the rears.

As to the proportioning valve, if you stay with the stock ABS, you're pretty much screwed. Changing out the booster and master cylinder will do nothing but allow you to move more fluid with less pedal travel, but will do NOTHING to change the pressures and brake bias as the proportioning valve controls that. And the proportioning valve is integral to the ABS unit, so you are stuck with whatever is available to work with it. You MIGHT be able to get a proportioning valve out of an 01 DODGE as I think DODGE kept the KELSEY HAYES ABS through 02, but they went to 4 wheel ABS and dual piston calipers in 01(prior to 01 DODGE used the SAME EXACT BRAKES as GM, they even BOUGHT the parts from GM). The newer GMT-800 trucks went one step furthur with the proportioning valve, and built it into the actaul hydraulic control unit, and it's not replaceable. Upgrading would also be out of the question as it needs J1850 data messages that the GMT-400 trucks do not send out over the data bus. And the GMT-900 trucks went even furthur with an electronic bias control to the proportioning valve controlled via the ABS. your problem is going to be you are going to probably need MORE rear brake pressure for the rear discs to really work(probably about 50% more pressure), and no external add in proportioning valve is going to give you this control. So you will be in the bone yards looking to see if the 3500 HD used the KELSEY HAYES 3 channel ABS like the GMT-400, or ditching the ABS all together, and getting an aftermarket proportioning valve installed.

 

[Big T]

OK we've established that the GMT 400 brake systems sends pressure appropriate for drum brakes in the rear. That pressure level is lower that what would be sent to a rear disc brake set-up such as on the GMT 800 HD trucks. So the likelihood of the ass end coming around to kiss the front end in a panic stop is off the table.

The remaining issue is a concern, based on system specs, that brake pedal travel may be negatively impacted by converting to GMT 800 front and rear disc brakes while using the GMT 400 master cylinder and hydro boost. So far, we do not have any actual experience on this, just concern based on size of pistons in the calipers and size of the master cylinder.

Finally, we're screwed on proportioning valve solutions because the later model ABS won't talk to our GMT 400 data bus.

The speculation is that we will need MOR rear brake pressure for the rear disc brakes to really work and there is no real solution for this.

As I've said, we're approaching this incrementally. Front brakes first. Observe pedal travel.

Plenty of people offering rear disc brake conversion kits for the GMT 400 and not a mention of proportioning valve, nor complaints about braking ability with those conversions. I will note they use a brake caliper from an '80s Cadillac Eldorado.

Like I said, we'll approach this incrementally. If the rear axle swap doesn't work, we'll put the old one back on.

 

[THEFERMANATOR]

I honestly do not know if it will work or not, that is the point I'm trying to make. For me it wasn't worth re-inventing the wheel to try it and see either. And the back end coming around and trying to overtake the front end IS a concern when you don't have enough rear braking. This was what the increase in rear brake pressure was for with the different proportioning valve as the wandering and pulling under braking with the back end trying to overtake the front end was because there wasn't enough rear brake pressure to keep it under control. Somebody needs to look into the GMT-455 trucks, and see if the ABS on them is the same, if so the proportioning valve from one of them could be swapped in to get a 4 wheel disc proportioning valve, or the 01-02 DODGE valve may be an option.

 

[WarWagon]

Highway/freeway driving is CA really isn't. It's a parking lot. If you can stay off the brakes for a long distance the bearing runout will kick the pads away from the rotors and the pads last way longer than if you just 'tap' the brakes every 5 miles on the freeway. This is not just the release of the brakes, but, moving pads away from the rotor that distance and bearing runout cause. It also improves MPG. Drum brakes do have the advantage of springs pulling the shoes off and away from the drum.

Then we got some grades like "The Grapevine"... Driving style and conditions make a difference. However 17K is about the life of 3/4 ton front brake and 1/2 tons of this era are 12K. So IMO the service life of the pads at 15K is in the ballpark.

Ceramic pads do last longer than semi metallic, but, when push comes to shove the semi metallic resist fade longer. IMO the semi metallic stop quicker with less pedal pressure. Pad quality and composition have a big effect on the system.

Having working rear brakes and changing up to carbon Kevlar metallic that have a high coefficient of friction certainly would reduce the work the front brakes have to do.

Next thing on the brake upgrade list would be 4 piston calipers and get rid of the flexi mush feel of the GM single piston calipers. Some of the movement you feel in the pedal is the single piston calipers flexing.

 

[THEFERMANATOR]

Highway/freeway driving is CA really isn't. It's a parking lot. If you can stay off the brakes for a long distance the bearing runout will kick the pads away from the rotors and the pads last way longer than if you just 'tap' the brakes every 5 miles on the freeway. This is not just the release of the brakes, but, moving pads away from the rotor that distance and bearing runout cause. It also improves MPG. Drum brakes do have the advantage of springs pulling the shoes off and away from the drum.

Then we got some grades like "The Grapevine"... Driving style and conditions make a difference. However 17K is about the life of 3/4 ton front brake and 1/2 tons of this era are 12K. So IMO the service life of the pads at 15K is in the ballpark.

Ceramic pads do last longer than semi metallic, but, when push comes to shove the semi metallic resist fade longer. IMO the semi metallic stop quicker with less pedal pressure. Pad quality and composition have a big effect on the system.

Having working rear brakes and changing up to carbon Kevlar metallic that have a high coefficient of friction certainly would reduce the work the front brakes have to do.

Next thing on the brake upgrade list would be 4 piston calipers and get rid of the flexi mush feel of the GM single piston calipers. Some of the movement you feel in the pedal is the single piston calipers flexing.

I have the biggest of the single piston calipers, and have no mush to my pedal what so ever. The stainless brake hoses make all the difference(and good pads seated in properly). As to the proportioning valve, I did some digging, and it appears the GMT-455's used the same KELSEY HAYES ABS that our trucks did, and they came stock with 2 piston calipers front and rear(at least later years did). So there is an available option for a proportioning valve that is calibrated for all disc brakes that will work. Only hurdle is finding a wrecked 3500HD to snag a valve out of.

 

[SnowDrift]

We have heard of horror stories, also, on Suburban brakes, but I put brakes on our '94 K1500 chassis at around 90,000 miles and didn't replace them until it was past the 200,000 mile mark. It just hasn't been an issue for us - to my knowledge, the proportioning valve is original.

Great thread, guys! It helps me to decide whether I want to take the disc brake plunge or not.

I look at Smithville's truck and wonder "if he ever chooses to sell this thing, I hope he has a list of what is what so the next owner doesn't have a nightmare" Then I think, "heck, why ever sell it? I have no plans to sell mine and it's not nearly as great of a truck as his"

 

[Big T]

Highway/freeway driving is CA really isn't. It's a parking lot. If you can stay off the brakes for a long distance the bearing runout will kick the pads away from the rotors and the pads last way longer than if you just 'tap' the brakes every 5 miles on the freeway. This is not just the release of the brakes, but, moving pads away from the rotor that distance and bearing runout cause. It also improves MPG. Drum brakes do have the advantage of springs pulling the shoes off and away from the drum.

Then we got some grades like "The Grapevine"... Driving style and conditions make a difference. However 17K is about the life of 3/4 ton front brake and 1/2 tons of this era are 12K. So IMO the service life of the pads at 15K is in the ballpark.

Ceramic pads do last longer than semi metallic, but, when push comes to shove the semi metallic resist fade longer. IMO the semi metallic stop quicker with less pedal pressure. Pad quality and composition have a big effect on the system.

Having working rear brakes and changing up to carbon Kevlar metallic that have a high coefficient of friction certainly would reduce the work the front brakes have to do.

Next thing on the brake upgrade list would be 4 piston calipers and get rid of the flexi mush feel of the GM single piston calipers. Some of the movement you feel in the pedal is the single piston calipers flexing.

Parking lot is an understatement. Rush hour is really rush all day, with the traffic bad even on weekends now.

I've got the braided stainless brake lines, stainless hard lines. September 2013 I installed new Raybestos Advanced Technology Slotted Rotors (great), Raybestos Advanced Technology semi metallic pads, Reman Calipers from NAPA. I did not write down the mileage at the install, but from March 2014 Smog Check to January 2016 Smog Check I out on 10,800 miles. Add another 6 months on to that back to the brake overhaul and you get around 14K miles and the pads were done.

We've got 3 other vehicles, including a 2014 4Runner and none have the brake wear that these Suburbans do. That 4Runner is my wife's and I got it for her as she wanted something smaller than the Suburban to go off-road in. She's put 50K miles on it since September 2014, which explains the lower use on the Suburban. Front brakes are dual piston calipers. When I inspected the front pads at 45K miles, I estimate they will go at least 70K miles. The braking power on the 4Runner is outstanding.

I'm interested in the proportioning valve from the GMT 455 - 3500 HD. Which years am I looking for? I'd just order new.

The last of the parts/tools just came in for the GMT 800 front brake conversion:

 

[SmithvilleD]

I think the early C3500HD's had a height sensing valve (limits rear pressure rise more when no load on rear, riding higher). Not certain if that continued when they got the Kelsey Hayes 4WAL abs system. BigT - I believe '95+ C3500HD's got 4WAL and a good search term is "combination valve." If not familiar with it, follow the brake lines from the MC to abs, they go into this valve block. They essentially let front/rear pressure rise equally until a "knee" point pressure, where it introduces more restriction & slows the rate of pressure rise (compared to front) on the rear circuit. They don't truly cap rear circuit pressure, but slow it's rise rate proportional to the front.

This link explains proportioning well:
http://stoptech.com/technical-support/technical-white-papers/proportioning-valves

I make the distinction (that the rear circuit pressure isn't capped), because functioning ABS, plus that continuing pressure rise is why/how so many big(ger) front brake kits get away without changing proportioning. When bias isn't idealized, at the point of front wheel lockup & ABS active, pushing harder on the brake pedal continues to raise rear circuit pressure & typically can continue until rear wheels begin to lock. The ABS is modulating the fronts while the rears are catching up. Not at all trying to say this is ideal, but I've driven some cars that I didn't think should have worked nearly as well as they did - eg., front brakes swapped from relatively small front discs to the 6 piston CTS-V Caddy calipers.

If the C3500HD combination valve can be found, it's logical to try it. With any of these brake combinations, still gotta try it & see how it works. One thing I'll do & see if it provides useful information, is take my truck/trailer loaded heavy down a nearby 1-2 mile grade, then pull over & IR temp all 4 rotors - temp is still relative to rotor size/heat dissipation ability, but will show if the rears are doing useful work or not. I do this on trailer brakes as well as bearing hubs as it's easy & can show if a bearing's running hot, or one brake is doing more or less work relative to the others.

Snowdrift - would be silly to sell my truck, but yes, I keep good records I enjoy doing custom/fab work & building the skills to do it (just started learning TIG welding) - I like my hobbies to incorporate some useful value although the vehicles typically get beyond what most would do.

 

[WarWagon]

I tried stainless braided brake lines on a 1995 1/2t Yukon and although improved it was still mushy. Other systems do just fine w/o SS hoses. Like say a 1999 Chevy 1/2 ton pickup.

GM was selling their SUV's so fast they didn't have to improve much on them in the 90's. There wasn't really anything else in it's class for a time.

I doubt that ANY of the other vehicles weigh as much as the Surburban including the extra diesel engine equipment. The pad size limits were covered above, but, it still comes down to things like how much work the rears are or are not doing. The newer 4 disc systems have larger pads larger diameter rotors and longer life in part to lower running temps.

I just read the other day that "smaller" brake systems were in part due to CAFE rules in the '70's. Not that you could lock up the 4 wheel drums on a specific full size Buick of the 50's...

 

[THEFERMANATOR]

Theres an article out there showing that the stock 2500 BURB proportioning valve does in fact cap the rears at 600 psi, this is why they had pulling problems under hard braking. The harder you hit the brakes, it only brought in more front brakes exaggerating the problem of theback end trying to pass thefront end. The updated valve increases said cap to 800.

 

[Big T]

Found the Suburban Brake TSB. Note the comments about having to file the steering knuckles to get proper caliper clearance. First I've heard of that. Sounds like a great overall engineering effort. Oh and I found this in a thread where the guy had wrecked his truck because of the crap brakes. Said he was only getting 8K to 10K miles out of the pads.

Caliper TSB

Tech - Brake Lead/Pull and Frt Brake Wear (Repl. Frt
Brake Pads, Rr Brake Shoes) #99-05-24-001A
Brake Lead/Pull and Front Brake Wear (Replace Front Brake Pads And/Or Rear
Brake Shoes)
1992-1999 Chevrolet and GMC C/K Cab Chassis, Crew Cab, and Pickup Models
with Power Brakes (RPO JB8)
1992-99 Chevrolet and GMC C/K Suburban Models with 8600# GVW (RPO C6P)
1996-2000 Chevrolet and GMC G Vans with Power Brakes (RPO JD7 & JB8) and
GVW's of 7,700# up to 9,500# (RPO's
C3F, C6P, & E23)
This bulletin is being revised to update information in the Models Section
and Correction Information. Please discard Corporate
Bulletin Number 99-05-24-001 (Section 5 -- Brakes).
Condition
Some owners may comment about the vehicle leading or pulling to either side
while applying the brakes, and premature front
brake wear, especially vehicles that have been used to tow a heavy load.
Cause
The condition may be caused by wearing of the front brake material.
Improvements to the above conditions can be gained
through brake material modifications. On the Suburban models, the best
performance from this new brake material can be
achieved with replacing the brake combination valve.
Correction
Replace the rear brake shoes with P/N 18029651. THIS FIX DOES NOT APPLY TO
THE 13 x 2 1/2 BRAKE,
DURASTOPT P/N 18029650, OR ANY OTHER SIZE. On Suburban models, a brake
combination valve must be
replaced. ALL OTHER MODELS (C/K, G Van), NO BRAKE COMBINATION VALVE CHANGE
IS REQUIRED.
Refer to the Drum Brakes or Hydraulic Brakes subsection of the appropriate
Service Manual. For G2/G3 Series Vans only
(with GVW's of 7,700# up to 9,500# -- RPOs C3F, C6P, E23) also replace the
front brakes pads with P/N 12471685.
Refer to the Disc Brake subsection of the appropriate Service Manual.
Important
The new front brake pad material has been selected for improved wear
characteristics. Some customers may experience
increased brake noise from this brake pad and should be advised that some
squeal is a characteristic of this particular material.
Important
If you encounter disc brakes that are extremely worn on the inboard side,
with very little wear to the outboard side, verify the
clearance between the caliper and the steering knuckle bracket stops.
MEASURE THE CLEARANCES INDIVIDUALLY AND ADD THE CLEARANCES TOGETHER FOR
TOTAL
CLEARANCE BEFORE REMOVING THE CALIPER. If the caliper (total clearance) to
the steering knuckle stops is under
0.26 mm (0.010 in), correct this by filing metal off at the steering knuckle
stops in order to obtain a total clearance of
0.26 to 0.60 mm (0.010 to 0.024 in). DO NOT FILE METAL OFF OF THE CALIPER.
Refer to the Disc Brake subsection
of the appropriate Service Manual for additional information on this
procedure.
Parts Information
Part Number
Description
Qty
18029651 (17452R Canada)
Shoe Kit, RR Drum Brk (13 x 3.5 in.)
1
12471685
Pad Kit, Frt Disc Brk
1
15650150 (92-94 Suburbans)
Valve Asm., Brake Comb.
1
12548265 (95 and Newer Suburbans)
Valve Kit, Brake Comb.
1

Parts are currently available from GMSPO.
Warranty Information
For vehicles repaired under warranty, use:
Labor Operation
Description
Labor Time
H0257
Shoes and Linings, Drum Brake - R&R or Replace (Rear)
Use published labor operation time
H0042
Pads, Disc Brake - R&R or Replace (Front)
1.0 hr*
H0780
Valve, Brake Combination - Replace
Use published labor operation time
*This time is updated from the current Labor Time Guide. The next issue will
contain this change

 

[Big T]

Article on the Brake TSB

When faced with a RWD vehicle
experiencing a lead or pull and
premature pad wear on one side you would
want to focus your attention on the front
brakes, right? Not necessarily. While it is true
these symptoms usually relate to a brake
imbalance caused by such items as a
restricted brake hose or caliper problem
this is not always the case. I recently had an
opportunity to work on such a vehicle. The
vehicle in question was a 1994 ? ton Chevy
Suburban. The vehicle had been experiencing
a lead or pull and premature wear for
over a year. All traditional fixes had been
applied with some of them being done twice
but to no avail.
This vehicle and others like it demonstrate
some important aspects of performing brake
diagnostics. The key aspects demonstrated
by these vehicles are:
• You must diagnose brakes as a system,
• There are exceptions to every rule, and
• In today’s times you must have access to
TSB’s.
GM issued a TSB for the 1992 to 1999
Chevrolet & GMC C/K Cab Chassis, Crew
Cab, Pickup and Suburban as well as 1996
to 1999 G Series Vans (TSB 99-05-24-001,
June 1999) that involves customer complaints
of a lead or pull that may be accompanied
by premature pad wear. Sound familiar?
The fix listed for all but the Suburban
models is the installation of a new set of
rear brake shoes. Yes, you heard it right,
rear brake shoes. The installation of the
new brake shoes changes the front to rear
brake balance of the vehicle shifting more
work to the rear brakes. The overworking
of the front brakes makes them very sensitive
to slight side-to-side differences which
may cause the lead or pull and the heavy
front bias causes the premature pad wear.
On the Suburban models equipped with
13” by 3-1/2” brake shoes the fix involves
not only replacing the brake shoes but also
the combination valve. Both of these steps
are aimed at directing more braking to the
rear brakes. With access to this information
we ordered the necessary parts for our
Suburban. Before replacing anything we did
some pressure tests on both front wheels
and from front to back. Not surprisingly, the
front side to side pressures were near identical.
This gave the front hydraulic circuit a
clean bill of health. The front to rear pressure
demonstrated why the Suburban needed
a revised combination valve. The combination
valve on these vehicles houses a
metering valve, pressure differential switch
and proportioning valve. The proportioning
valve is designed to prevent the rear brakes
from locking up during a panic braking situation.
Apparently someone made a mistake
on the Suburban models
because the valve
on these vehicles is
cutting the rears off
way too soon causing
the front brakes to be
overworked and over
sensitive. Typically, a proportioning valve
will start to work at about 600psi. Front and
rear pressures will stay the same up to this
point and then start to differ with the fronts
continuing to climb and rears being limited
to somewhere
between
650 to
800psi. The
proportioning
valve on
the
Suburban
we were working on began limiting rear
pressure at about 400psi and stopped it at
around 600psi.
After getting the new parts we installed the
revised rear shoes and the new combination
valve. We left the pressure gauges
attached so we could take pressure readings
with the new valve installed. After finishing
installing the parts we pressure tested
the system. With the new valve installed,
the front and rear pressures stayed the
same to just over 600psi. The rear pressure
stopped climbing at about 800psi. Both the
starting point and ending point represented
a significant difference from the original
valve. This change, combined with the
revised rear brake shoes, had a dramatic
impact on the front to rear brake balance
of the vehicle and solved the pulling problem.
We must assume that the repair will
also improve the front pad wear.
Here is a case that all indicators point to a
problem with the front brakes. The demonstrates
there are exceptions to every rule and
why you have to look at the entire system
when performing brake diagnostics. Not having
access to the TSB leaves the shop to try
and come up with the solution on their own
which in this case would be near impossible.
Checking for TSB’s should be a routine part
of the brake diagnostic process. The earlier
this is done the better.
The part information for the vehicles
involved in this TSB is as follows:
Correction: Replace the rear brake shoes
with P/N 18029651. This fix does not apply
to 13 x 2.5” brake shoe, the Dura Stop P/N
18029650, or any other size brake. On
Suburban models, a proportioning valve
must be installed. All other models no proportioning
valve is required.
®
AUTO PARTS
BRAKE LEAD/PULL & PREMATURE
WEAR ON GM TRUCKS & VANS
by Bill Williams
for Mighty Auto Parts
© 2001 Mighty Distributing System of America • Norcross, GA 30092
FOR INFO ON MIGHTY PARTS: 1-800-829-3900
mightyautoparts.com

 

[SmithvilleD]

I stand corrected.

May have found part numbers for a disc/disc combination metering/prop valve designed for the Kelsey-Hayes EBC310 ABS the combination valve we're discussing mates to. Looks like there were some GMT400 bodies Police Tahoe, P30 van disc/disc, and maybe some GM motorhome vehicles that used this ABS.

Haven't had time to search a lot, but believe these part numbers relate to the combination valve they used: 12548278, 172-2243.

 

[THEFERMANATOR]

I stand corrected.

May have found part numbers for a disc/disc combination metering/prop valve designed for the Kelsey-Hayes EBC310 ABS the combination valve we're discussing mates to. Looks like there were some GMT400 bodies Police Tahoe, P30 van disc/disc, and maybe some GM motorhome vehicles that used this ABS.

Haven't had time to search a lot, but believe these part numbers relate to the combination valve they used: 12548278, 172-2243.

The GMT400 police TAHOE got the 2500 front brakes, but as far as I know it still used drum's for the rears. There might have been some P30's transit buses built with 4 wheel ABS though, who knows on motorhomes. I thought the motorhomes didn't get ABS from the ones I've seen. There is the C3500HD that did come with disc/disc, and I believe it was dual piston calipers front and rear at that.