it may just need to be taken apart and cleaned, crud builds up and deforms the seals. Mine leaked all over the bodywork and just simply taking out the piston and wiping it off cured it. If there is corrosion more drastic steps may be necessary.
+1.
My experience has been that it's the rear master cylinder that is most likely to have corrosion, and the front to fall victim to crystallized fluid. I suspect that the rear, being at the low point of the system, has more water collecting there, not to mention road spray.
Since the front is at the high point, the water is going to collect at a lower point, probably the calipers. The heat there might help to boil it off and make the brakes spongy before it can cause corrosion problems.
