Brakes activating on their own

[steamship]

Originally Posted by dangerdred

what has me baffled is the problem rectifies itself with a few deep pushes on the brake pedal.

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.

[dangerdred]

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...

[steamship]

Originally Posted by dangerdred

Does this indicate air still in the system?

Originally Posted by dangerdred

what has me baffled is the problem rectifies itself with a few deep pushes on the brake pedal.

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?

[dangerdred]

Originally Posted by steamship

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.

[steamship]

Originally Posted by dangerdred

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.

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.

[ainarssems]

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.

[dangerdred]

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.