MG Car Repair: Sticky brakes, sticky brakes, flat blade

Question
Hi, I recently wrote in with a brake problem which you correctly identified as a leaky master cylinder.  Once I replaced that, the problem went away.  However, while in the process, I also replaced the rotors, which were worn unacceptably thin.  Now the front brakes never really disengage.  They provide slight resistance at all times, especially the left one, and when I went for a drive, they got HOT.  Since the pistons were presumably positioned for too-thin rotors for a decade or so, would corrosion and dirt have built up to prevent them from moving in the proper range?  Can I clean this out or do I need to replace the calipers?

Thanks as always.

Answer
The pistons inside the calipers are made of steel (which can rust) covered with a thick, hard layer of chrome to protect them and allow them to slide smoothly.  If they have rusted excessively, they will not fully retract and the pads will drag on the rotor.  

Remove one brake pad, and insert a wide flat blade to prevent the piston from coming out too far.  Operate the brake pedal to pump the piston out a bit (say 5mm) and then clean the exposed edge.  Then push the piston FULLY back into the calliper using the blade as a lever.  Insert the brake pad and operate the brake pedal to settle the pad.

Do the same for all 4 pads.  If you still have a problem after this, then you need to replace the callipers.  If you are really handy you can strip and rebuild your old callipers with new seals and pistons.  By the way, for a little extra expense you can now get stainless steel replacement pistons.