[subxero]

if it is moisture i am thinking it is a problem more up top in the master than down low at the caliper. The heat from braking would have warmed up any fluid lower in the system. Since the braking did not get better as things warmed up and remained the same it is logical to think that this section would not be the issue as you do not have the same issue on warmer days.

However the master cylinder will not see the same heat as caliper ect... which could explain why you did not see an improvement after things heated up at the caliper.

^is any of this making sense?

But on warmer days everything including the master are at a more reasonable starting temp and things are all fine and dandy or so it would seem.