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Safety Questions about Installing C3500HD (disc/disc) brake booster and proportioning valve onto K2500 Suburban (disc/drum)

The Burb

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Hi, Everybody-

I've read lots of amazing forums here about upgrading GMT400 Suburban brakes using K3500 DRW and C3500HD parts (special shout out to THEFERMANATOR, Big T, and Pistachio Bob). My question is whether using a brake booster and proportioning valve from a 1997 C3500HD, which was a disc/disc set up, would present any safety issues in my '98 K2500 Suburban which is a disc/drum arrangement. For context, I will be installing new K3500 DRW calipers, pads, rotors, wheel cylinders, shoes (Durastop per bulletin); a 1997 C3500HD brake booster, master cylinder, proportioning valve, and the curly steel brake lines connecting the MC to PV; and also putting in braided SS lines front and rear.

As much as I want massive stopping power for my behemoth Burb, my first priority is that this brake overhaul will not put my family in a likely dangerous braking situation given the disc/disc vs. disc/drum compatibility. I would appreciate your expertise and input before I for sure pull the trigger, thanks!!
 

JayTheCPA

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Welcome to TTS!


Unless you already have the disc parts and need an excuse to do the swap, might want to consider sticking with drums.

Even though some have gone with pads in the rear, my vote is stick with the shoes. When I bought the Burb I looked into a disc conversion and the suspension shop recommended to keep the drums as they had more braking force due to a larger surface area. Definitely not looking to start a shoe / pad battle, and am just passing along information.

While I cannot answer the question about how a disc / disc system handles the brake upgrades, I do have experience with disc / drum and the upgrades. I adapted the K2500 Burb's brakes to the 1 ton dually system (1 ton front rotors + calipers, proportioning valve, and rear cylinders (already had the larger shoes)) and life was much better. After the swap to 1 ton parts, I saw no need to put in the extra effort of a disc conversion as the Burb's brakes worked very well in all conditions. Even when hauling the family, plus gear, plus ~8K# on the ball. Wet or dry. Normal stop or panic.

The only downside to shoes is that they do need adjustment. But after the 1 ton conversion, the automatic adjustment actually worked and all I had to do was a few hard-stops in reverse after each oil change and they reset themselves. With the OE brake parts, somebody had to crawl under the truck and spin the star.

While not part of the question, consider adding spacers in the rear to make the rear wheels follow the front wheel's track. This made a noticeable improvement with straight-line handling. Best time to add the spacers is while doing brake work :)
 

THEFERMANATOR

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Welcome to TTS!


Unless you already have the disc parts and need an excuse to do the swap, might want to consider sticking with drums.

Even though some have gone with pads in the rear, my vote is stick with the shoes. When I bought the Burb I looked into a disc conversion and the suspension shop recommended to keep the drums as they had more braking force due to a larger surface area. Definitely not looking to start a shoe / pad battle, and am just passing along information.

While I cannot answer the question about how a disc / disc system handles the brake upgrades, I do have experience with disc / drum and the upgrades. I adapted the K2500 Burb's brakes to the 1 ton dually system (1 ton front rotors + calipers, proportioning valve, and rear cylinders (already had the larger shoes)) and life was much better. After the swap to 1 ton parts, I saw no need to put in the extra effort of a disc conversion as the Burb's brakes worked very well in all conditions. Even when hauling the family, plus gear, plus ~8K# on the ball. Wet or dry. Normal stop or panic.

The only downside to shoes is that they do need adjustment. But after the 1 ton conversion, the automatic adjustment actually worked and all I had to do was a few hard-stops in reverse after each oil change and they reset themselves. With the OE brake parts, somebody had to crawl under the truck and spin the star.

While not part of the question, consider adding spacers in the rear to make the rear wheels follow the front wheel's track. This made a noticeable improvement with straight-line handling. Best time to add the spacers is while doing brake work :)
He's asking about using the booster, master cylinder, and proportioning valve ftom a c3500hd which had 4 wheel discs on his suburban which has disc front and drum rear if I understand him correctly, not converting to rear discs. GM used a massive 1 9/16" piston in the 3500hd with 4 wheel discs vs the 1 1/4" piston that went into disc/drum trucks with hydroboost brakes, so it is a MASSIVE step up in fluid moved per pedal movement. Even the nbs & nnvs trucks only used a 1 7/16" bore master with 4 wheel disc & hydroboost brakes.

GM used the same proportioning valve in the 3500hd as they did all other abs units according to the parts catalogs. My concern would be a very sensitive pedal because of the massive upsize in master cylinder bore size.
 

The Burb

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Thanks, JayTheCPA, TTS is awesome and glad be on. So I'm actually sticking with the disc/drum set up and have bought all the 1 ton dually swap-in parts (so great to hear you've been please with yours! I can't wait for mine). Because GM/AC Delco discontinued making the updated GMT400 Suburban/Tahoe brake proportioning value, I learned from THEFERMANATOR that the C3500HD proportioning valve will shoot higher PSI to the rears (just how much PSI I have not read yet, I know 800 PSI was what the updated Burb/Tahoe value shot back), so snagging one of them was my only option for higher PSI to the rears. Also, a C3500HD brake booster and master cylinder are simply massive. I've acquired all 3 along with all the 1 ton dually swap-ins.

The unknown for me is whether too much pressure will be shot to the rears (never thought I'd have that concern with a GMT400 Suburban!) Will this present safety issues? The C3500HD hydroboost brake booster was for a disc/disc set up and has the tell-tell blue nitrogen accumulator as opposed to the gold found on disc/drum hydroboost GMT400s. I've read the blue has 450 PSI of nitrogen (vs 350 PSI in the gold) but what effect having this C3500HD brake booster, together with a C3500HD proportioning valve and MC, will have on a disc/drum set up is my pressing question.

[Not to de-rail the more pressing question above, but could you provide more details on the rear spacers? You've piqued my curiosity]
 

The Burb

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Just saw the FERM had posted while I was drafting my reply to Jay. Thanks, FERM! Appreciate all the input and clarification, everyone, keep it comin'.
 

JayTheCPA

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For the wheel adapters I went with 2" on each side. Got them here: https://www.wheeladapter.com/

Best way to figure out whether spacers are necessary is to measure the width of the front tires and compare to the rear.

In my case I had the common 4" difference.

On concrete pavement, there really was no difference.

On blacktop highways where the roads developed a bit of rut, the spacers made the Burb more stable and more like it was on concrete. Without the spacers, the Burb would surf a bit as it decided where in the rut it wanted to ride.
 

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FRANKENBURBAN
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Thanks, JayTheCPA, TTS is awesome and glad be on. So I'm actually sticking with the disc/drum set up and have bought all the 1 ton dually swap-in parts (so great to hear you've been please with yours! I can't wait for mine). Because GM/AC Delco discontinued making the updated GMT400 Suburban/Tahoe brake proportioning value, I learned from THEFERMANATOR that the C3500HD proportioning valve will shoot higher PSI to the rears (just how much PSI I have not read yet, I know 800 PSI was what the updated Burb/Tahoe value shot back), so snagging one of them was my only option for higher PSI to the rears. Also, a C3500HD brake booster and master cylinder are simply massive. I've acquired all 3 along with all the 1 ton dually swap-ins.

The unknown for me is whether too much pressure will be shot to the rears (never thought I'd have that concern with a GMT400 Suburban!) Will this present safety issues? The C3500HD hydroboost brake booster was for a disc/disc set up and has the tell-tell blue nitrogen accumulator as opposed to the gold found on disc/drum hydroboost GMT400s. I've read the blue has 450 PSI of nitrogen (vs 350 PSI in the gold) but what effect having this C3500HD brake booster, together with a C3500HD proportioning valve and MC, will have on a disc/drum set up is my pressing question.

[Not to de-rail the more pressing question above, but could you provide more details on the rear spacers? You've piqued my curiosity]
The blue one is 600 psi and the gold is 400 psi. Got this info from a shop that rebuilds the accumulators. What is odd is not all 3500hd boosters got the blue accumulator. Mine has a gold one on it. I have the 3500hd booster but the standard 1 1/4" master cylinder on mine now. It gives a very noticeable increase in assistance over the regular hydroboost.
 

The Burb

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The blue one is 600 psi and the gold is 400 psi. Got this info from a shop that rebuilds the accumulators. What is odd is not all 3500hd boosters got the blue accumulator. Mine has a gold one on it. I have the 3500hd booster but the standard 1 1/4" master cylinder on mine now. It gives a very noticeable increase in assistance over the regular hydroboost.
Here is a pretty good link I found to explain what the accumulator actually does (cuz I sure didn't know)--turns out it is a safety backup to give you a few pumps of assisted braking in the event the hydroboost fails. https://www.brakeandfrontend.com/operation-diagnosis-and-repair-of-hydro-boost-power-assist-systems/ So having the blue accumulator with disc/drum equates to a few extra pumps in a dire situation than the gold would, I guess.

Per The Ferm's comment about possibly having a very sensitive pedal with the C3500HD BB, MC, and PV in place, does high brake sensitivity necessarily equate to too much pressure? And thus drums building up too much pressure to locking up? Seeing Ferm's got everything I will minus the C3500HD MC and PV and hasn't had locking up issues, I would hope I'm in the clear. Obviously, I'd prefer to know before hitting the road, but this may be a "first of it's kind" swap (yours truly!). I'll be sure to report back once it's all said and done, wish me luck. Any final words?
 

JayTheCPA

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If the Burb has a functioning ABS, am not seeing rear wheel lock at speed as much of a risk, if any. I do see the concern in too much bias going to the rear. If anything, the most noticeable difference is that with the rears grabbing harder, the Burb will either ride flat while braking or maybe try to squat in the rear a bit. This is in comparison to the OE setup which has the Burb wanting to nose-down when braking as the rear brakes are not doing much work.

Will defer to Ferm for a better answer or correction though.
 

Twisted Steel Performance

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@The Burb , If you could, post the part numbers for the HD hydro boost and M/C, and any other HD parts you found, the HD parts are hard to locate on most sites, I have a rear disc kit for my dually that I will be starting the install in the spring..
I know some part numbers are on this site but it's hard to find them, maybe if we can compile a list it can be added to the stickys...
 

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I can lock mine up and send the abs into a tizzy no problem. I will probably be putting the old booster back into mine one day. That said, pressure is dependant on how hard you mash the pedal. Going to the bigger booster and master will increase the assistance to move the pedal but also greatly decrease pedal travel. As to the accumulator, it works by providing a reduced pressure charge through basically a backup side of the booster. The higher pressure accumulator simply helps to give greater assistance, not more pedal actuations. The power steering pump supplies 1200-1500 psi, so you can see how the accumulator gives assistance, but less of it.

I'll be going with either gmt800 booster or an 11 booster when I do the brake upgrades on my Tahoe.
 
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