how do I release the piston on my brake master cylinder?

IT DEPENDS HOW BADLY IT IS SIEZED UP, FIRST WAY IS,REMOVE THE CALIPER,REMOVE BRAKE PADS,PUT A PEICE OF WOOD BETWEEN
THE PISTON/PISTONS AND THE BODY OF THE CALIPER,MAKE SURE THERE IS ABOUT 6mm/1/4" play so the piston can come forward,start pumping the brake lever until you get some resistance,if it moves put a thinner peice of wood in and pump again,you might aswell take it right out to clean it up,with very light grade emery cloth/paper and some thin oil, wd will do,everything must be cleaned inside the caliper and the outside of the piston,wash in methylated spirit or some spray on brake cleaner,when reassembling put clean brake fluid on the piston,if the piston doesn't move out you can use a blowlamp to heat the caliper body but be careful not to burn any rubber covers or set the brake fluid on fire,make sure its empty.
hope this works for you because its worked for me many times.
Den

den
April 2008