diagnosing front disc brake issue

[WagonCrazy]

The Setup:
57 wagon with front discs/rear drums
CPP power front disc conversion kit (2 inch drop spindles,10 inch rotors with single piston calipers)
8 inch vacuum booster
CPP dual mastercylinder (front disc, rear drum)
CPP "auto" proportioning valve properly plumbed in.
New rubber brake lines installed. Still holding tight...no leaks.
New steel brake lines installed all the way around 8 years ago.

This brake system has been installed and working just fine for about 8 years now. Haven't had to mess with it.

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.

First Diagnosis/Fix:
Removed and Disassembled the 2 front calipers, then cleaned up the pistons and inner surfaces. Nothing was scratched/marred/worn BUT there was a sort of hard shellac on portions of the piston surface, and what looked like a little rust shellac'd on them.
Reassembled, added brake fluid, bled the system (via wife inside on the pedal).
Brakes worked fine for about 10 to 20 minutes of city driving, then did the same thing.

Ive never drained the brake fluid since installing 8 years ago.
Ive never run the system out of fluid...always kept it topped off.
Dot 3 used.

Next up:
Thinking the Master Cylinder is sticking...caused by 8 years of crud, or some moisture, rust, lack of driving...sitting in the driveway, etc.

Gonna pull it, clean it, inspect it for leaking seals, etc. It's a cast iron corvette style MC that CPP sells with the clip on lid.
Gonna drain ALL fluid from the system, and bleed at all 4 corners after bench bleeding/installing the existing MC (after I clean it).

Should I screw around with fixing the existing MC, or replace it?
Would the proportioning valve malfunction to the point of causing intermittant fronts to lockup/release/lockup/release quickly?

[chevynut]

Are you sure it's the front brakes acting up? What makes you suspect them and not the rear brakes?

[WagonCrazy]

With the windows rolled down, I can hear it coming from the fronts.
What makes you think it might be the rears?

Either way, both are controlled by the push of a piston thru the mastercylinder, so my thinking is that if both fronts are locking up, then its not an individual wheel caliper causing it...it's something in either the master cylinder or proportioning valve components.

gonna pull it apart on Sunday morning this weekend. Will snap some pics and post after that.

Either way, the whole system is getting bled and new fluid put in.

Just curious if you guys ever experienced this, and what you found was the faulty component.

[Bihili]

Sound like fluid getting past the seals in the MC.

[rockytopper R.I.P 5-13-2017]

May not be related but several folks lately including my friends 55 had issue with brakes locking up and it was the booster causing it. Apparenently they have a defect that can cause them to apply brake pressure and not release. They say the ones that are chromed are worse because they get disassembled at chrome shop and not properly reassembled. Apparently there are only a hand full of manufactures making them so all the suppliers are having issues not just CPP.Might be worth looking into. It was so common last year that I bought Cnut 's bracket and went hydraboost to eliminate the booster altogether.
Rocky

[Maddog]

That's a new one, never heard of those symptoms before. I would suspect booster or prop valve, probably would change booster first, they're cheap enough.

[chevynut]

Paul, just because you think you're hearing it in the front brakes doesn't tell me that's where the problem really is. The front and rear brakes do interact inside the MC and the prop valve, if it's one that senses both front and rear pressure. I can't think of any defect that could cause what you're describing. If the booster was "locking up" it seems like you would have the brakes on all the time. I don't know what could make the brakes turn off and on like you're describing, but if it was me I'd isolate the problem by disabling the front or rear brakes to make absolutely sure where the problem is. Then I'd work on troubleshooting it from there.

I guess I can imagine drum brake shoes chattering in the drum before I could see disc brake pads chattering. That's what makes me think it might be in the rear.

[Maddog]

Those Chicom CPP master/booster/prop valves are about $180 on fleabay. Just buy another entire unit, replace it and your problem will probably be gone, no need to try to play Sherlock Holmes.

[chevynut]

Here's some stuff on brake vibration. Not seeing this in person is kinda hard to diagnose. If it's a vibration at high speed, it could be warped rotors. I had a vehicle that did that when the brakes got hot and you could feel it in the pedal. Do the rotors look like they're uniformly worn? Any burn spots? Does it go away after the brakes cool down?


What is Brake Vibration?

Brake vibration is where a shaking motion occurs when the brakes in a car or other vehicle are deployed. This can vary from a slight shaking to a quite severe shuddering, depending on the severity of the condition. It can also be known as rotor shimmying or brake pulsation.

What causes it?

If the brake rotors installed on the vehicle have more than .05mm of run out (where the brake rotor moves from side to side more than a minimal amount of .05mm), then brake vibration will occur through the steering. Usually, this is due to one of two different reasons. The face of the disc may have some rust or dirt on it, which is causing the run out figure to be incorrect. Such build up may occur during the lifetime of the rotor, and this can cause the run out to be up to five times the limit installed by the factory. If the problem is not due to this, it may be that a ‘thin spot’ has occurred on the rotor (where there has been intermittent pad contact instead of a constant motion). When the brake pedal is pressed this causes the brake vibration to occur. This is technically known as Disc Thickness Variation, or DTV for short.
As it is essential that your brakes run true, this must be sorted out as quickly as possible.

How to rectify the problem

The first thing to do is to determine whether it is the front or rear rotors that are causing the brake vibration. If the steering wheel itself is shaking, it is more likely to be the front, whereas if the whole car and bodywork shakes, it will probably be the rear. Additionally, it is not necessarily the pair of rotors that are the problem; it may only be one of them. Finding the location of the shaking is the first step to solving the problem, however.

[rockytopper R.I.P 5-13-2017]

The folks that had booster problem had similar symptoms. After the car is driven for a short time the brakes lock up. The vaccum is apparently leaking and applies brake pressure or does not release after applying the brakes. The car can apparently set for a while and brakes work for a short time then lockup again.