TSB for Proportioning valve
I had mentioned this in 97Suburban's thread on his rear brake problems, but I figured I'd post in a new thread as this is quite lengthy and it doesn't really pertain to his problem, anyway - didn't want to hijack!
Anyways, I did call my local dealer and he confirmed everything in the article.
Sorry for the awkward reading - I'm not sure how to post pdf files. I've also rad that while this does alleviate the problem, it's not always a cure-all.
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
