Are there bulletins from GM on how to correct a problem that supposedly doesn't exist? I read that repair requires the removal (or modification) of a part in the brake combination valve or master cylinder. That information came from an individual who had the work successfully done at the local GM dealership. Searching GM's website, calling, asking local dealers, and much discussion did not produce any listed remedy. GM either does not want to acknowledge that rear brakes are my problem or couldn't care less!
I currently know of three trucks ('97-99) with the exact same brake problem! In my research I have found this to be a common complaint for these years. My last resort is to scrap my present system and adapt the brakes from a '75 Chevy pickup. Please help me, and thanks for the answers. I really enjoy your monthly column.
A: We have to think GM does care that you have brakes. Unfortunately you didn't give us much information to go on. Is your truck a 1/2-, 3/4-, or 1- ton Silverado? Yes, there are a few service bulletins (SBs) around the scope of your problems. We'll refer to a few of them and make some guesses based on what type of vehicle you have.
First, you stated that you have intermittent ABS engagement at slow speed. SB 02-02-25-006B explains a corrosion problem that occurs between the hub and the ABS sensor mounting. What this causes is an increased air gap between the sensor and the reluctor wheel on the hub. This generates a lower-than-spec voltage signal that goes back to the ABS computer and applies the ABS erratically. The fix is to remove the ABS sensor, plug the hole in the hub to prevent debris from falling into the hub, lightly sand the mounting surface of the hub, apply Rust Penetrating Lubricant PN 89022217, and allow it to dry. Then apply a thin layer of wheel bearing grease to the hub surface and the sensor O-ring to prevent further rust buildup under the sensor.
As for the prop/combination valve, the only bulletin for that is on '92-99 Suburbans with an 8,600-pound GVW. Another thing you may want to look at if you have a 3/4- or 1-ton truck is that some models have a brake bias valve connected through linkage from the frame to the rear axlehousing. This adjusted the rear braking pressure based on the load in the box of the truck. If this valve is malfunctioning or maladjusted, it could be causing reduced brake pressure to the rear brakes.
We hope this helps with your problem. Also, just because there isn't a bulletin on your problem doesn't mean you don't have a problem with the combination valve on your truck. It may be limiting the brake pressure also. Check it out and be safe.
Technical questions for Kevin McClelland can be sent to him at firstname.lastname@example.org.