diagnosing front disc brake issue

[chevynut]

So perhaps with the booster or MC causing the brakes to drag on the rotor the heat warps the rotor and causes the chattering/pulsating/"skipping"? Lockup is one thing, but locking uip, releasing, then locking up again doesn't sound like the same thing. My Bronco's brakes were usually pretty smooth but when they got hot they pulsated like crazy. It was a warped rotor (or two) that caused it. It does sound like the brakes are getting hot.


"The Symptom:
Brakes working fine for the first 15 minutes of driving (when cold).
Then about 2 weeks ago while running around town, hit the brakes and the fronts BOTH "skip". Rapidly. They lock up, release, lock up, release, etc. Very fast, as if I had antilock brakes. Let off the pedal, and brake again...same thing. Then they don't do it for 10 or more minutes afterwards. Intermittant!
No pull to one side or the other....both wheels seem to lock up and release quickly while "firmly on the pedal". Don't really feel it in the pedal at all (like you can with antilock brakes). Brake pedal is firm.
But after one of these events, I smell hot brake pad and the brakes feel like they "faded"...can feel it in the pedal."

[WagonCrazy]

Update:
There never has been any shimmy in the disks when braking....hence I doubt warped rotors. I've had several newer cars do that (after excessive mountain pass braking) so I know what that feels like. Had none of those symptoms.

This morning, I removed the master cylinder and proportioning valve. Here's a pic just before the wrenches came out.
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When I removed it, I checked for fluid getting past the seals...None to be found. Dry...
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So then I drained and disassembled it. Other than some varnished looking stuff near the seals, it was fairly clean for being untouched for 8 years now.
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Cleaned it all up, inspected and everything seemed to look fine. So then I turned to disassembling the CPP proportioning valve. Same thing...not terribly dirty nor filled with evidence of gunk or water...
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So I cleaned it up, and reassembled it. Then decided (since I had the MC off the booster) to do the "putty test" to make sure the booster pin was sitting tight to the MC pin.
A round dab of plumbers putty stuck on the booster pin, then hold the MC tight to it, then check the clearance.
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Then reassembled the brake lines, filled the chambers (bench bled it before installing it) and used one of them Harbor Freight "one man" brake bleeding tools (with air hooked up to it) and bled at all 4 corners, starting with rear passenger, rear driver, front passenger, then front driver).
Hopped in an did a test drive for 1/2 hour around town and IT DIDN'T REPEAT THE SKIPPING from before.

So...I think I had dirty fluid, crud in an orifice somewhere that prevented adequate brake fluid flow when pressing and holding the pedal during normal braking. Seems to be gone now.

Before I did all this today, I had a friend tell me last night (experienced mechanic for a living) that he thought it may have been the booster going bad...allowing vacuum to slip out. He suggested pulling (and sealing off) the vacuum line from the engine, then driving it without the power brakes to see if it repeated the skipping again. That was good advice I think, but I went the route of looking into the MC and prop valve and flushing the whole system with new fluid.

Think it's fixed now...

[Rick_L]

I've been reading this with interest.

I suppose that dirty fluid that has a lot of water in it could do this (the fluid must be boiling when the symptoms occur), but there must have been some other factor (like something making the brakes drag or stick) that added to the problem.

Anyway, good news that you apparently have corrected the problem.

Should be a word to the wise to change out the fluid every 3-4 years even if there's no problems/symptoms - as is recommended but few do. (Mental note to self, need to do that on my Nova.)

[chevynut]

I have to say the proof is in the pudding, but I'm skeptical that the root cause of the problem is actually fixed. Time will tell, I guess. It seems like bleeding the brakes the first time should have flushed most if not all of the old fluid out of the system, but maybe not. Too bad we will never know what the real problem was since you cleaned everything up before testing it again. You did hit all the important things you needed to check.

Anyhow, glad it seems to be fixed.

[Rick_L]

If you bleed the brakes and quit when you see no bubbles, you are a long way from flushing and replacing all the fluid in the system.

[WagonCrazy]

I bled more than a quart of fluid thru all 4 lines, till it was very clear and no bubbles. I'm sure I have all the old fluid out of the lines. They are only 1/8 inch lines...so not much volume.

I could have just started by bleeding the lines, but I think I would have ended up pulling and inspecting the MC and prop valve anyways, so I just chose to get it over with at the start.

[chevynut]

Hope you solved it Paul. I'd just out of curiosity like to understand what the problem was and why the brakes were pulsing off and on. I'm having a hard time seeing how "bad" brake fluid could make that happen.

[WagonCrazy]

Well guys, today I went for the one aspect of the brake system I haven't looked into ...the rear brakes.
Bingo...

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Lining of the shoe(s) was coming off. Yes...it started to squeel when driving it this morning. That was the final clue...

I installed these rear shoes 4 or so years ago, and don't remember where I got them, but they are CHEAP JUNK. They are "bonded only" to the metal shoe. So they let loose with less than 5000 miles on them.

New shoes installed are better quality with rivits holding the material to the metal shoe.

So I got a round trip accomplished all thru my brake system, only to find it in the last place I looked.

Didn't Laszlo ask me that question at the top of this post??????

Dumbazz Paul.