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Thread: TSB for Proportioning valve

  1. #1
    Join Date
    Jul 2003
    Location
    Allentown, PA
    Posts
    1,355

    Default 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
    1998 K2500 Suburban 6.5L TD 3.73 rear, Ron Schoolcraft 18:1, Kennedy ECM & IC, Timing gears, Splayed main caps, 3.5" Kennedy Exhuast/No Cat, K&N Filter, Boost/Tranny Temp/EGT(Pre Turbo), Ceramic-coated Manifolds, 195 Stat's, 265/75's (VSSB Adjusted) 7,000lbs (on a scale) Remote Mount Oil Filter, Remote Oil Pressure Sensor

  2. #2

    Default

    Dennis,

    Found this thread today. I am having the same issue with my 98 K1500 Suburban. I have been replacing pads TWICE a Year!. The rear driver side shoes, when I pulled the drum, showed no signs of wear. There was oil on tme so I thought that was the cause. I replaced pads and shoes after replacing the wheel bearing seal on the rear axle -this sucked because you have to open the differential to get to it.

    Now new pads again- shoes not wearing... WTF?!? I check the forums and see the TSB mentioned. I got the new combination valve and am awaiting the new shoes.

    I had a question about the new valve and wondered if you can give any tips on how to replace it?

    Thanks in advance.

    Ed

  3. #3
    Join Date
    Jul 2003
    Location
    Allentown, PA
    Posts
    1,355

    Default

    Hi Ed,

    I never did get the new valve so, no, unfortunately I can't offer any tips or advice. I'm tentative to tear into this because one of my rear bleed screws is broken off and I can't turn the other (afraid to turn too hard as it might break), so I wouldn't be able to successfully bleed the brakes when done. Actually, what I'd really like to do is swap the entire axle for a newer one (from a salvage yard) with disc brakes.

    Have you adjusted your rear brakes manually?

    There's a lot of knowledgeable people here on this board, so I'm sure someone can offer more help.

    If you do get this done, please report back as I'm sure I'm not the only one who would be curious to hear your results.
    1998 K2500 Suburban 6.5L TD 3.73 rear, Ron Schoolcraft 18:1, Kennedy ECM & IC, Timing gears, Splayed main caps, 3.5" Kennedy Exhuast/No Cat, K&N Filter, Boost/Tranny Temp/EGT(Pre Turbo), Ceramic-coated Manifolds, 195 Stat's, 265/75's (VSSB Adjusted) 7,000lbs (on a scale) Remote Mount Oil Filter, Remote Oil Pressure Sensor

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