Hi, I have the exact same problem with right front brake sticking on and being unable to push back the piston unless the bleed valve is loosened. Replaced seals in caliper first as thinking piston was jamming. I did suspecr the ABS unit as my issue occurred after doing an emergency stop where brakes seemed to have remained locked on. I turned the ignition off and car became mobile again except for issues with right front brake was overheating. This indicates that the ABS unit is the culprit so will get a replacement from the local wreckers. Thanks for the posts as it was very helpful.