I had to have the braking system pressure bled after replacing the rear pads as couldn't get any pedal resistance and hence no brakes. After this there was pedal resistance but the brakes began to come on and gradually the brakes came on harder and harder (as did the pedal) until they locked on. I have now had all the Calipers and Master Cylinder replaced to no avail. If I release the pressure by bleeding at the Calipers or Master Cylinder this solves the problem temporarily as does destroying the vacuum by separating the hard hoses by the Vacuum Pump. I've had numerous people look at it without being able to determine the cause. I have noticed a slight oil leak around the Vacuum Pump on the end of the Cam. Could the Vacuum Pump be at fault or the Servo, or maybe the ABS Pump (where is it)? Seems to be the only options left, I'm at my wits end, if it wasn't such a faultless car previously with a fantastic engine that I didn't mind spending a little on because I aim to keep it for a few more years I'd have scrapped it by now!

There wouldn't be any need to bleed the brakes just by replacing pads. If whoever bled the system has done it via the pedal it is possible they've damaged your master cylinder. Why did you have it bled after changing the pads?

As Meaty says, there is no need to bleed the brakes just because of replacing pads (even with new discs). And yes, "bleeding" by pumping the pedal (or even using a pressure bleeder) can damage the master cylinder seals, but that should give you no brakes, not brakes locked on.

 

If oil is leaking into the vacumn pump, that could cause the brakes to come on.

 

There is no "ABS pump" as such; electronic ABS is a set of electronically controlled hydraulic valves that apply/release brake pressure to the wheel cylinders, but the peak pressure applied is normally pedal pressure * servo advantage, and the ABS should stop the brakes ever locking on.

 

As ever VCDS is your friend, since at worst it can eliminate the electronics.

maybe the servo faulty however breaking the vac pipe to release, suggests the one way valve doesn't work either-- not sure on yours,   but some have a small valve also near the vac pump has been known to coke up