My pads were very free and smooth in the contact areas so no amount of filing was needed.

 

I'll save CWARD and Mike saying it again, but I thought that too. It has cured my problem.

 

What have you to lose by trying it? Your car, your choice though.

 

Good luck.

I've a niggly feeling that something else is applying some pressure to the caliper whilst the car is in motion.

Basics of the braking system are incredibly simple, ABS adds some complications, but under normal conditions the fluid pressure is distributed evenly to all four calipers. Only the caliper and piston size determine the pressure on the disc and the sue of the disc creating more leverage.

If one caliper isn't performing the same as the others first look at cleaning up all the contact points and ensuring the floating pins are corrosion free and lightly greased (too much causes suction through a vacuum effect). Reassemble with copper slip on the back of the pads to prevent brake squeal and on the pad slides to allow free movement. The pads should held in place and not tight as the backs will expand slightly with heat then tight becomes stuck.

Second thing to look at if still not fixed is changing brake fluid and bleeding the system. Old fluid absorbs moisture which causes corrosion, lowers the boiling point which all effect the pressure on the piston.

Third is to replace the seal and clean up or renew the piston.

Fourth, there is no such thing as a seized piston only lazy mechanics that will fit a new caliper rather than service the existing one which is done at your expense. This only leaves the master cylinder and ABS which require more specialists skills and tools. Steps 1 to 3 only costs your time and effort and no more than £50 to replace the brake fluid, seals and piston.

Gremlins don't really exist and methodical fault finding isn't magic.