Failing brake master cylinder?

[melomandn]

So I'm not sure whats going on so heres some background.

A couple weeks ago my rear brakes started to seize up and it trashed my rear rotors and pads so I cleaned up the rear sliders, checked out the rear brakes and made sure the ebrake cable wasnt getting stuck and up until last night my problem was solved.

Now all four of my brakes are sticking, I'll be pushing 15 psi in fourth just to get above 45mph.
Heres whats weird about it though, the brakes will get so bad like that and the steering wheel will start to shake and I can stop on an incline and the car wont roll at all, but then if I pump the brake pedal 5-10 times they will release and I'll be able to drive the car normally again.

This morning I left my buddies house, brakes acting like they should, not sticking at all and then halfway home I had to pull over and try to pump my brakes to get them to release because I could hardly move.

After the last incident I've stopped using my ebrake and just park the car in gear because I thought that was the problem but clearly its something bigger.

So I need some help here because I just did the A8 brakes up front and dont want to trash those too.

[bman005]

Drive around the block a little bit and once it starts acting up jack the car up and see which wheels are stuck. Its either a caliper or brake hose issue most likely. Typically when your master cylinder fails your vehicle will have trouble stopping, not moving

[melomandn]

Its all 4 wheels, the cars basically undrivable now

[redline380]

Won't be master cylinder. If that goes or the brake boost goes you're brakes would be spongy and wouldn't work very well at all. I would maybe look at abs system. Try unplugging an abs sensor or disabling it by pulling a fuse. If your brakes don't stick after that its an abs issue.

[melomandn]

Won't be master cylinder. If that goes or the brake boost goes you're brakes would be spongy and wouldn't work very well at all. I would maybe look at abs system. Try unplugging an abs sensor or disabling it by pulling a fuse. If your brakes don't stick after that its an abs issue.

Thanks! I didnt even think of that, will have to go give it a try

[bman005]

The ABS system doesn't apply the brakes on its own so it won't be that. It's a mechanical issue somewhere.

[melomandn]

Where should I be looking? I literally got stuck on the middle of a hill last night, just planted there...

[mysman]

If you fried the rear brakes, you may have destroyed the caliper seals. In two weeks of winter driving, the caliper pistons may have become corroded and stuck. The question is what caused the original seizure? It sounds like it occurred right after you installed the A8 rotors with what, the TT brackets?

[walky_talky20]

Hmm. This is a pretty interesting problem. If it truly is locking up all 4 wheels, it is pretty unlikely to be caliper issues. And I highly doubt the ABS module would or could continue to apply brake pressure with the key off. I think the brake pedal pumping (in order to release them) is quite significant here.

When the pedal is pushed, it closes off the master cylinder connection to the reservoir and begins to pressurize the lines. As the brake pedal is released, the pressure is lowered. When release *fully*, the pressure will "dump" back into the reservoir as that passage is reopened. This guarantees full pressure release. HOWEVER, if the pedal is not adjusted properly (ie: the rod length), it may never return enough to "dump" the pressure. Some residual pressure will remain in the system. This adjustment may be completely on the edge of working such that everything is ok when it's cold, but as temperatures of various components increase - it no longer dumps. Thus leaving the car immovable until somebody plays with the brake pedal repeatedly.

That is really my best guess for what is going on here. I think there is a high probability that the brake pedal rod is adjusted improperly, especially considering that the pedal cluster was changed or modified during the 5-speed swap on this vehicle. If this is the case, the procedure to correct this is known as the brake pedal "free play adjustment".

If the rod length looks good, then I would move forward to some testing. Basically, you've either got a problem with "rogue" hydraulic pressure: brake fluid pressure remaining with zero pedal input, OR calipers/pads/what-have-you that are hanging up at each wheel. A good way to figure out what you're dealing with here is to get it into the failure mode, then crack a caliper bleeder. If you have pressure there: BAM, hydraulic problem.

[melomandn]

Thanks walky, I've had my 5 spd swap done for about 6 months now and havent noticed any problems, even slight ones dealing with the brakes and now I cant even move my car its so bad so I'm feeling like if it was the brake rod I would have noticed something earlier, and not all at once.

I did crack a bleeder when the car was "stuck" and it shot out brake fluid so its a hydraulic problem, so all I can think of is the master cylinder.

If it helps at all heres a little more info... if i let the car sit for at least 12 hours it will work as normal the first couple miles, but then after driving it will start to get worse and worse until I can hardly move.
So basically it gets progressively worse while driving, but then when let sit its back to normal.
Not sure if these symptoms help but I'm desperate for some help because I've been without my car for almost a week now and this is my DD.

[melomandn]

Walky, could you explain some more about what the brake rod may be doing, and whether or not I need to tighten it or loosen it to fix my problem?

I replaced the master cylinder and my problem went away for about 200 miles then it came back so then I unplugged my abs (electronically) and the problem has not yet came back and is showing no sign of coming back. Im still curious about the brake rod though as sometimes the brakes do seem as if they are beginning to stick.

[Harrisonjra]

any update on this?
my 2003 allroad with a manual conversion is having EXACT symptoms, I've changed all four soft lines to stainless, new master, power bled, brakes are great until car warms up.

[walky_talky20]

You have to make absolutely sure that the free-play adjustment is correct. When you release the brake pedal and it hits the stop, the master must be 100% released. If it hits the stop and the master rod is held in - even a little bit - you will have trouble with the brakes hanging on. The rod is very, very adjustable and correct adjustment of the rod is absolutely critical for safety.

[seanf86]

I had this exact problem on my b6, one day driving my brakes started applying and it go so bad trying to get the car off the road my rotors turned blue from the heat, I found when this happened cycling the key and restarting the engine fixed it when the abs does the self check at start up, replaced abs pump with a used unit and no problems since

[redline380]

I had this exact problem on my b6, one day driving my brakes started applying and it go so bad trying to get the car off the road my rotors turned blue from the heat, I found when this happened cycling the key and restarting the engine fixed it when the abs does the self check at start up, replaced abs pump with a used unit and no problems since

So it was the ABS system? All hail Redline!

[seanf86]

So it was the ABS system? All hail Redline!

Pretty sure it was, after cycling the key and I heard a pop the pedal was no longer hard but it took out my master too, after changing both it was fine for the last 3 years.