Brakes activating on their own
 

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?

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.

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.

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.

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...

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?

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.

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.

Now that is weird, and would tend to suggest that it isn't the same issue I had. Since you've had the brake master cylinder replaced, that only leaves the ABS unit, but how the problem manifests and resolves is a real mystery. I wonder if there is some Basic Settings that can be run on the ABS unit to see if everything is working properly. A real head scratcher.

Did a scan and got no ABS fault. Will try checking fuses and voltages to the ABS unit tomorrow to see if it turns up anything, hopefully dodgy wiring...

Was doing some searching, so just read through your posts again. We can assume you've eliminated the master cylinder and the ABS unit (no errors) as being the cause. I came across the following thread (post #4 in particular), and they mention about air still in the system. The mystery is your second quote above, how a 'few deep pushes' rectifies it. Note that the thread isn't about an Audi, but the principle should be the same:

https://turbobricks.com/index.php?th...riving.268140/

BTW Did you run the procedure in VCDS to bleed the ABS unit through the NSF caliper?

Thanks for all the help.

I did nothing with VCDS other than scan for errors. I thought the pressure bleed with the engine running was enough to push fluid through the ABS pump, no?

I'm starting to think it may be the ABS module or maybe pump, as the pressing of the brake pedal almost down to the stop a few times when stopped at lights etc. is no longer working. It's very strange, today I drove 6 miles into the city centre, and another 6 miles back home again, and brakes didn't misbehave once. I then take the car out this evening and I haven't gone more then 100 yards from the house and I feel the brakes coming on. The other thing I thought I noticed this evening when they locked - I pulled in and put it in Park and pumped the pedal a few times, then tried it in Drive or Reverse and saw it was locked solid no matter how much pedal pumping I was doing. So I put it into Neutral and killed the engine. Within about 10 seconds I started to feel the car roll so started it up and was able to drive on. I'm tempted to buy one of those ABS pumps/modules off €bay although as I think about it now, if I can get my trusty ASZ Bora back on the road (busted sump) I'd probably be better off taking my own pump off and getting it reconditioned with a warranty etc.

To be honest, I have no idea how these ABS systems work, so I was just following the instructions given in the relevant PDFs about bleeding the brakes, which included the procedure to bleed the ABS unit itself, after bleeding all the individual brakes.

It does sound as if the ABS unit is maybe on its way out, with the way it first locks the brakes and the way you could unlock them, but odd that no codes are shown.

Buying one off eBay is probably the 'cheapest' option, but you need to ensure that you get one with the exact same part number. The downside to this is that you're buying a 15 year old part. Your suggestion of getting your existing one reconditioned with a warranty will be more expensive, but worth it in the long run.

Is it still braking all wheels or mostly just one? Can you see pressure reading in VCDS when it's happening? If it's all wheels and you can see pressure, considering you already replaced master cylinder, I would look at vacuum booster/servo, may be it's getting stuck. If it's mostly one wheel then look at the caliper, take the pads out, give contact surfaces a good clean removing dirt and rust, push pistons in and out couple of times and make sure they move freely, the grease and reassemble.

Was the master cylinder new or used, if used it could be faulty as well, even if new but been sitting for a long time on the shelf in damp environment it may have experienced some corrosion.

As for the ABS pump bleeding, if there was some air getting in it, it will probably need bleeding with VCDS.

It's a bit too late for it now, but when I replaced master cylinder, after putting new on, I did not want to mess around with bleeding as nipples can be corroded and stuck so I pushed the pistons on each caliper in to force brake fluid back up in master cylinder and reservoir bleeding air out the top. I was not sure at first if it will work but seemed to work very well.

I pulled both front wheels off and OS disc is a lot more scored than NS so will replace OS caliper in next few days as I already have one that I never fitted.


EDIT: No brake issues since OSF brake caliper replaced.

I had bought this replacement caliper initially after first experiencing the issue as it seemed to be the OSF wheel that was locked and hot. What had put this idea out of my head was subsequently at some other time getting out to feel both front wheels and both seemed fairly equal in heat. So I had never fitted the replacement caliper until now...ugh.