I found this article a while back and kept a copy. If the mods need to edit it, have at it.....
I haven't done any of the mods yet but I thought I might post it for your interest.... It seems to be related to this thread.
It doesn't address the rear lock-up, but it does discuss the wear and pull.
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 toTSB’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
W
Part Number Description QTY
18029651 (17452R Canada) Shoe Kit, Dura Stop Brake 1
(13 x 3.5 – Suburban)
15650150 92-94 Suburbans Valve Assembly – 1
Brake Combination
12548265 95 & Newer Suburbans Valve Assembly – 1
Brake Combination
$ 100.00 each but for towing, they were worth their money. :thumbsup:
