Questions and Answers

Buzzing GMC K3500 Q. Hi. I am trying to diagnose a problem with my 1997 GMC K3500 6.5 liter turbo diesel single (drum) rear wheels with front disc brakes. I get an occasional mechanical buzzing vibration when depressing my brake pedal and an odd fade in the pedal- (stepping / sinking.) The vehicle stops okay, and the brakes were inspected when I bought the truck used with 96,000 miles.

It doesn't happen all the time, but it is troubling when it does. The dealer (a local garage) put in new front ABS sensors and front pads, but I attributed the problem to the vehicle having tires of three different brands. (All four tires were the proper size, but the o.d.'s differed from mfg. to mfg.)

I have since replaced the odd tires with a set of four matching OEM size tires, and the problem got better, yet it still occurs. I don't think it's a problem with the ABS, but I suspect it may be the hydro-boost unit or the accumulator- if it has one?

I can't be certain of my suspicion because the brakes work fine when it doesn't act up. Is there a simple way to disable the ABS system to see if this vibration will still occur? That would at least rule out the ABS. Have you heard of this before? Can you offer any advice?

Al

Some afterthought clarifications, that "buzzing" I spoke of is felt only AT the brake pedal, nowhere else. Also, I know the truck has a hydroboost unit, but I don't know if it has an accumulator. I hope this helps. Thanks.

A. What you need to do is to drive it on some wet pavement and do a panic stop to get the feel of a ABS application. This will probably answer your question. You may be loosing traction or you may have a sensor somewhere that is going to zero at times and is making the ABS system think it's slipping/skidding. What you're describing sounds to me like a ABS application. You will here the pump run and feel some feedback in the pedal. Also may have a leaking wheel cylinder or axle seal in the rear.