Brakes

[Inspector]

Something odd is happening with the old 93. The brakes seem to work fine untill just before stopping. At that time the brakes seem to fade and act as though they had failed and then return. The power steering works just fine through all this. The stopping power is good untill just before stopping. No antilock light comes on.
Any ideas folks?
Denny :confused:

[Inspector]

C'mon folks sombody has had to have run into this before.
Denny

[DmaxMaverick]

Inspector

You are probably not getting a response because what you describe is not by any means common. I've never heard of it. Congratulations....you found a new bug....perhaps.

I hate to see some alone in their time of need, so I'll respond with what may be less than helpful info.

The only things I can think of would be ABS or hydroboost. Engine rpm and steering wheel input can/does effect hydroboost performance. Is there any difference in pedal feel? Can you hear/feel the ABS pump? Are there any leaks? Does it seem to effect both front and rear brakes? The effect of front/rear could help with troubleshooting. If it effects both, you can rule out the wheel cyl's/calipers. If it effects only one, it probably is at the axle level. There are some issues involving the change of tire diameter or gear ratio and the ABS.

Another posibility (really reaching here) could be glazing on the pads/rotor that may decrease the effectiveness at certain rotational speeds (harmonics). Perportioning valves do fail, but I've never heard of one causing your issue.

Good luck. Be sure to let us know what turns up. This could be a good one.

[Inspector]

Thank you Dmaxmavrick:
This one has been going on for a time. The brake pads are good and no rotor warp. The tires have been changed but the issue exsisted before that. I have towed all over heck with a heavy trailer and never experienced this. It always stopped straight and fast. Never so much as a hint of a fade. Now solo it gets a bug. I should get a chance this week to check it out further.
Thanks again
Denny

[Dimsdale]

I would be thinking hydroboost too, but you would have to have a pretty severe drop in RPM, and the pressure accumulator would have to be defective.

Do you get any boost at all with the engine off? The accumulator should give you a couple of assisted brake applications before it gets really hard to apply them.

Otherwise, I am stumped!

[GOT DSL]

I had a similar problem with my brakes. Turns out the accumulator lost its charge.
You can check as follows;

With engine off push on brake pedal 3 ~ 4 times, this will drain the booster of power steering fluid. If the accumulator is bad it will be loose on the booster. The accumulator is the brass looking cylinder on the inboard side of the booster.

[Inspector]

If it is the accumulator, can it be changed out seperate from the booster? Or will I have change out the entire booster.
Thanks for the reply.
Denny

[GOT DSL]

I changed out the entire booster and master cylinder only because I thought that the parts were getting old. I considered drilling a hole in the accumulator and screwing in a schraeder valve so I could pressurize it myself. The rebuilt booster that I bought had this set up. The accumulator was 140.00 at my local Chevy dealer. The rebuilt booster and master cylinder cost me 200.00.

[schamp]

You might try disconnecting the antilock wires at the front wheels and see what happens. My 97 suburban was doing simular thing. I have mine disconnected and have solid brake pedal. Brakes on these things not to good to start with. You can also hook up a scanner and turn the antilock off. Scanner will then indicate that truck is ok to drive. In your case if it doesn't work, reconnect the wires. Might be worth a try.

[Inspector]

This truck only has anti-lock on the rear. We will disconect the module and see if that will take care of the problem but doubt it. I'm thinking accumualator.
Denny

[Inspector]

A little udate on the brake issue. I drove it and sure enough just before stopping you get a little release in the peddal but the the brakes don't fade just a 1/2 second release and a good stop. It is a bit disconcerting. You can hear a quiet tic tic tic when the peddal releases in that split second.
The brakes look good. The anti-lock system seems to be ok. No codes. The accumulator is tight with the brakes pumped down.
Don't know where to go from here except some very expensive experimentation.
Denny

[BETA]

It is one or both of the front wheel speed sensors that don't signal properly because of a cracked induction wheel. You have to replace the whole wheel bearing assembly. This is a fairly common problem. $200 and 1 hour in the shop will cure it.

[Inspector]

Beta:
Thanks for the come back. This truck only has rear wheel anti-lock. I don't know how it senses wheel lock up in the back as I can see no wireing or sensors on the axel.
Denny

[rjschoolcraft]

The ABS uses the VSS signal for the rear wheels.

[Inspector]

Thanks Ronniejoe:
That answers that. Now if I could figure out the rest of it.
Denny

[StephenA]

What's the VSS signal?

[nopower65]

Have you looked at the brake fluid? Is it clear or direty? Brake fluid attracts moisture, and is degraded after it gets hot (frequent occurance if towing, since GM brakes are so inadequate). flush the brake fluis-- FLUSH it completely-- not just bleed-- and then see what you pedal feels like.

The symptoms you describe are similar to experiences with dirty/worn out brake fluid.

This will only cost you your time and about $10 in brake fluid. To be safe- the brake fluid should be changed annually.