This is a weird one, for me anyway.

225K miles, the front brakes act like I hit ice when I apply the brakes hard. Almost like a clunk thenjust fades like crazy. It started when I would go over a bump, but now it's anytime I use the brakes hard. Doesn't pull left or right, just acts like I hit a patch of ice. No chance of that here this week anyway.

Master Cylinder?
Proportioning valve?

Any ideas?

 

I don't even pretend to know anything about brakes.

I doubt it's the proportioning valve though, one would think that would only affect the rear brakes.


phox

 

That is kinda what I thought.

Maybe the master cylinder?

 

So....you have ABS right? Front and rear, or just rear? Does it act like the ABS in engaging? That is how I take it based on your comment about how it "acts like I hit a patch of ice" . There is more than likely a sensor or two going out and making the computer think the ABS needs activated. Will probably need a scan tool to know for sure which sensor it is. Has there been any work done to the truck lately? Hub bearings, rear end work? Just trying to get a good "history".

 

I have ABS, but I think it is the rears only. It has never worked, they will lock up. I have an ABS light that comes on occasionally when I am working on the brakes, not lately thought.

I just replaced the rear differential speed sensor a month ago. Nothing else worked on out of the ordinary.

Are you talking about scanning for codes? I have a scanner / Smarty.

 

Well, if you have 4WABS you can watch all of the speed sensors and IF 1 of them is dropping out ( reading 0 ) before the rest of them that could cause it. Does the pedal pulsate or buzz a little when it happens? Or does the pedal just sink? If it just sinks it could be the master cylinder. When it sinks can you pump the pedal once or twice and get it to come back up? Rear shoes being out of adjustment can also cause a low pedal. I would start with a basic inpsection and look for any seeping at the rear wheel cylinders. Then make sure the front brakes are o.k. and no sticking calipers and then check if it is just RABS or you can PM me your vin and I can get a build sheet to know for sure.

 

No buzzing or pulsating like ABS, it just "breaks loose" under heavy pressure.

The truck has only rear antilock, and it has never worked.

 

O.K.....is the fluid full? When it "breaks loose" does the red brake light come on on the dash? Does the pedal come right back if you let up and push it again?

 

No lights, breaks come right back to normal.

 

You can try to bleed the brakes. There could be some air in the rear (or front)that is allowing the proportioning valve to bypass some. Usually though, the light will at least come on a little when that happens. The only other thing I would have to guess could be the master cylinder leaking past the seals internally.

 

Ive got this exact same problem. I dont hear any tire skid so Im assuming that my fronts are just maxing out and the weight of the truck is too much for them.
Im going to get desperate enough soon to try some of those blueprinted calipers and rotors from EGR.
I have RWAL too and it doesnt seem to do a whole lot.

 

Check all your hoses, and see if you have a busted hose. That happened to me once. Just a thought.

 

I fixed it! It was the master cylinder. It must have been "busting loose" and allowing fluid to slip by (and not leak out) under heavy pressure.

 

I had the same problem, the piston gets to a ridge in the cylinder and leaks fluid past the seal...

 

So tell us, in order to fix the problem, did you do something to the exsisting master cylinder and did you replace it with another one?