1998 A4:

A local student called me today (needs to leave tomorrow 6 am for 8 hour drive to college).
He is too far away for me to hook up VAGcom today.

His brakes are locking up after a few minutes of driving!

He already changed:
- ABS pump
- ABS module
- Master brake cylinder

Any ideas what might be causing the system to build pressure and HOLD the pressure?

I advised to take the ABS module connector off (hoping that this will stop any gremlins that tell the system to BUILD pressure).

ANY QUICK HELP, SUGGESTIONS, TIPS much appreciated.

 

I had this happen to me and it was the brake pedal itself. The "nut" that connects to the rod was loose and after tightening it up everything was fine again.

 

Interesting, will check.

But how can that loose nut keep the hydraulic system under pressure?
How can it build pressure on its own (foot not touching the pedal)?

QUOTE=meloman;1419797]I had this happen to me and it was the brake pedal itself. The "nut" that connects to the rod was loose and after tightening it up everything was fine again.[/QUOTE]

 

Are all of the wheels locking up, or just one? I've had just the latter on some old cars I've worked. The internal liner in the flexible brake lines can collapse and behave as a one way valve. It will seem to bleed fine, but when you press the brake pedal, the fluid cannot go back.

To diagnose the problematic wheel, jack it up. Press the pedal a couple of times to build pressure and then open the bleeder. If fluid comes shooting out and the hub is able to rotate again, you've found the problem.

 

[/QUOTE]

For me, everytime I pushed the brake pedal down it wasnt coming back up all the way because it wasnt tight enough, thus not letting out all of the pressure each time and because of that it built up and all 4 would stick.

Is the master cylinder still good?

 

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If pads been ground down and rotor also ground far enough it can extend the piston can just past lip and get stuck. I've had this happen on other car/truck. Should that be the issue rebuild the caliper and have someone who knows how to QA machined work verify the column piston rides in is clean and meets height spec. Or get new one. That would keep the pressure on and lock up a wheel.