Brakes locking on

[vinnoh]

I have a 2005 2.0 fsi that has an ongoing braking issue. Previously it was a brake pedal dropping on its own volition, or staying partially down when breaks where applied.
Suggestion was that it could be the electric vacuum assist pump being faulty, and I had no way of really verifying this. So off to the shop it went . Two weeks have passed and in frustration I have taken my car back, supposedly fixed, but pulling left. Possibly a failed or failing caliper.
On driving it, the brake pedal appeared hard and yes it did pull a little to the left. The next morning I have taken it out to test the brakes. Not a good thing, brake pedal really hard and brakes appear to be staying on.
I did suggest that the vacuum assist may be faulty, and was advised this would be checked. So diagnosis was faulty brake booster, and possibly master cylinder. These have been changed out and pedal height issue appeared to be resoled. Was told in no uncertain terms that vacuum assist is working ok.
It did have new discs and pads on the car.
I would be grateful if anyone has some clue as to what may be the problem.

[Crasher]

With the engine off, pump the brake pedal half a dozen times and then whilst holding the pedal down, start the engine and the pedal MUST sink.

[vinnoh]

Okay, did that and all is good from that perspective. I have a distinct feeling that the calipers may be stuck or at least two are. Really overheated when I took it out to test them. (one front one rear) I will have a mobile mechanic come out and sus the system out. I really hate paying someone for a job and it to be not properly done. I would do it myself, but health issues are stopping me going anywhere near it.

[Crasher]

Is the front/rear feeling diagonal or longitudinal?

[vinnoh]

diagonal front near side rear off side. I have got the car to a reputable brake specialist to see what he can find.

[vinnoh]

I was just sitting here thinking about these brakes. The ABS sensor and ring is on the rear off side, could that be playing havoc with the ABS module and giving me the hard pedal?

[Crasher]

The ABS is very sensitive to poor signals from the hub pulse rings and you would get ABS or ESP warnings. The system is plumbed diagonally leading me to suspect the system has not been bled correctly. A common one we see is failure of the rear calipers, especially the right rear by about a 10:1 ratio, but it tends to be salt water corrosion from the millions of tonnes of the stuff they spread on our roads in winter. If people had decent tyres as appropriate we wouldn’t need so much of the stuff like in Germany where winter tyres are law. In OZ I don’t imagine salt on the roads is a problem...

[vinnoh]

It appears at the moment that the repairer (previous) has used a mismatched booster and master cylinder. The shaft between the two appears to be to long, so the spacing is out between the two. Hopefully today we get a proper diagnosis and solution.

[vinnoh]

The latest is that it is definitely a faulty booster both the replacement that was put in being the wrong one and the original eventually failed on a bench test. So a new unit is on the way and if all goes well I will have my car back at the end of the week. Has only been five weeks......so far

Okay. Final update.

Original brake booster faulty, with brake vacuum pulling brake pedal down, and or stay down after brakes applied. Pedal would return after 20-30 seconds albeit slowly.
Unfortunately the brake discs and pads where compromised by the constant application and heat and where replaced as well.