Brakes activating on their own

[dangerdred]

Car has developed an issue with the brakes, where it can't be driven more than a few miles before the brakes start to activate on their own, gradually the force increasing to eventually lock the wheels.
While driving, first thing noticed is gearbox taking ages to shift through the gears(was at 3k revs at one point before it shifted), and as the brake drag increases, car needs to be pulled over. Another clue that the issue is about to occur, is that the brake pedal seems to tighten right up, with barely any travel needed for application of the brakes. This increases to eventual zero travel at the pedal and the brakes locked on solid.
Every time it's happened so far, if I turn the car off for twenty minutes or so, and then check the brake pedal, once I can detect some travel in it, if I then start the engine, putting it in gear will confirm if the brakes have indeed unlocked their grip on the discs - enough usually to get me home. Car will then drive normally for some random period although the intervals have gotten shorter and shorter to the point now that the car is not really driveable.
I did get ABS brake pressure sensor failure error at one point, so I replaced that and all was good, until the issue returned. I'm thinking my ABS pump or module is failing?

[ainarssems]

Probably master brake cylinder getting stuck and not releasing properly all the way, had it on A6. Brakes are dragging a bit, heat up, brake fluid expands and presses brake pads harder and heat them more and so on until pressure increase so much you cannot drive. ABS sensor was probably OK, it was just showing pressure when the car was not expecting any.

[dangerdred]

Originally Posted by ainarssems

Probably master brake cylinder getting stuck and not releasing properly all the way, had it on A6. Brakes are dragging a bit, heat up, brake fluid expands and presses brake pads harder and heat them more and so on until pressure increase so much you cannot drive. ABS sensor was probably OK, it was just showing pressure when the car was not expecting any.

This may be the second time you've saved my bacon if it turns out the replacement brake master cylinder replacement resolves my issue!

The problem I'm facing now after replacing the part is that I can't seem to find any definitive process for bleeding the brakes after putting in a new master cylinder. Does anyone know if I'm right in thinking I connect up a pressure bleeder at 2bar and just start bleeding the calipers F/NS, F/OS, R/NS, R/OS and that's it? Or does brake pedal need to be pushed at any times, or do I need to do anything with VCDS?

I have tried bleeding them already starting at R/NS thinking it's furthest away, although the other issue I've encountered is that the small yellow half-turn cover on the brake reservoir is leaking badly when I apply pressure. Not sure if I need a new reservoir.

[steamship]

In terms of the bleeder kit, it needs 'at least' 2 bar, so keep it around 2.5 to be sure. I just keep an eye on the gauge as I bleed it, and then pump it back to 2.5 for the next one.

Instructions start on page 110 of the following PDF:

17-Service Manual Brake system.pdf

There are instructions immediately after this about the brake master cylinder. If there's any possibility of air getting into the ABS unit, there is VCDS procedure for bleeding that through the front left caliper. If running this procedure, make sure there is plenty of brake fluid in the pressure bleeder, as it runs for 10 seconds.

[dangerdred]

Originally Posted by steamship

In terms of the bleeder kit, it needs 'at least' 2 bar, so keep it around 2.5 to be sure. I just keep an eye on the gauge as I bleed it, and then pump it back to 2.5 for the next one.

Instructions start on page 110 of the following PDF:

17-Service Manual Brake system.pdf

There are instructions immediately after this about the brake master cylinder. If there's any possibility of air getting into the ABS unit, there is VCDS procedure for bleeding that through the front left caliper. If running this procedure, make sure there is plenty of brake fluid in the pressure bleeder, as it runs for 10 seconds.

Thanks, I'm reading lots of conflicting reports many of them saying just connect the pressure bleeder at 2 bar, start at the caliper furthest away and bleed working back to the nearest caliper and no need to do anything with VCDS. And at the same time, the factory instructions mention similar only starting at the F/NS caliper. I'm hoping my failure up to this point is down to the o-ring failure (which I've now replaced) in the reservoir preventing proper build-up of pressure.

[steamship]

Originally Posted by dangerdred

...just connect the pressure bleeder at 2 bar,

The only reason I'm aware of the pressure issue is because MikkiJayne said to have 'at least' 2 bar, as it needs 'more' than 2 bar to get through the ABS unit, and that's what the manual states.

[dangerdred]

Got the pressure up to 2 bar and bled from R/NS, R/OS, F/NS, F/OS with the engine running, and I now have brakes again. Was pretty hairy trying to move the car around with no brakes! Just need a new pressure bleeder now as the one I had couldn't really handle the pressure with lots of fluid escaping from the connection above the reservoir cap.
Just need to wait now and see if the master cylinder change has cured my brake binding issue...

[dangerdred]

Car is driving again with intermittent brake issues with brakes starting to come on slightly in the front - which is happening fairly frequently but not enough to have to pull over, and then again it mightn't happen for miles. Any times it happens, pressing hard on the brake pedal a few times will resolve it but then it might occur again 100 yards up the road. When I check the heat in the wheels after a drive I have - NSR is almost cold, OSR is warm, OSF is hot, and NSF is warm. Does this indicate air still in the system?

[steamship]

Originally Posted by dangerdred

Car is driving again with intermittent brake issues with brakes starting to come on slightly in the front.

Since the master cylinder replacement hasn't eliminated the problem, perhaps the OSF caliper is the problem. I had issues with my NSF caliper many years ago going out for a run one day. Country roads so only speeds of 40-50mph, but I noticed that the car just seemed to be working harder and harder. Stopped and discovered the NSF had near enough seized. Had to leave it for about half an hour for it to cool door, and then with some help from someone that I stopped outside their house, we managed to force the piston back. Thankfully I was less than 10 miles from home, so just a cautious drive back, and replaced the both front calipers.

[dangerdred]

Thanks yes, I thought it might be the caliper maybe not moving on the sliders or something but what has me baffled is the problem rectifies itself with a few deep pushes on the brake pedal. So the way it is now, I just pump the brakes at every opportunity when stopped. So the master cylinder or the subsequent bleed has resolved the original problem of the brakes coming on to the point where no further forward progress was possible at all until, like you said, wait for twenty minutes for the system to cool down. It's the ABS pump that sends the braking force down to each caliper is it?

I'll do a scan on it tomorrow to see if I can see anything.