vtraudt

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, TIP much appreciated

mtroxel

I'd check the condition of the slide pins on all corners.

vtraudt

They lock on their own (driving for a while, brakes start to bite until they lock up; not touching the pedal).

Release pressure, drive away.

mtroxel

Wow. So he's driving and without touching the brake pedal the brakes lock up?

David.Norton

I had that happen, different Audi, a 5K with the hydraulic boost. I had replaced the booster for leaking, and the rebuilt would just lock up while I was driving without touching the pedal. Very disconcerting. For that one I thought maybe they had shaved too much metal off of the mounting surface, so I shimmed it off, right as rain after that. Anyway, this is a different design, but I still think that you might be dealing with a booster problem. You might have him pull the hose and see if the problem clears up, keep in mind the car will be hard to stop, so try this in some quiet streets. If it does clear up and is just hard to stop time for a new booster.

x19

???? You didn't have any code(s)?????? x19

tommyleea

Was any maintenance performed prior to the issue?

vtraudt

Another 'locking brakes' case (2002 Audi Allroad 2.7T):

We replaced the master brake cylinder.
Still not sure what caused it, but assume dirt.
The entire process was:
- car came in with working brakes for auto/manual swap
- brakes not touched, other then connecting line for clutch to/from brake reservoir
- clutch not working
- replaced (used, came with manual tranny) clutch slave with metal slave
- clutch not working
- put working brake master on.
- clutch and brake working
- clutch slave line popped out. Replaced with metal braided one with secure screw type fittings
- clutch and brakes working
- 2 weeks later, brakes locking up
- removed brake master and replaced.

I assume that dried up brake fluid (from old clutch slave and clutch line) may have found its way into the system, causing the problems.

Prospeeder

If the pedal is adjusted too tight, it will not fully return the master, and as everything warms up and expands the brakes start locking on tighter and tighter since the master piston is not fully returned and allowing expansion. Try loosening up the adjustment on the brake pedal and see if it stops locking up.

Also the brake pedal switch and be adjusted to far out and cause the pedal to stop before fully returning.

x19

Add to above, try to loosen up nuts on B/Master cylinder when you got a problem then you will know what is next? Good luck. x19