Power brake conversion issue

I wouldn't assume it is the master cyl., it works fine cold for how long? As SIX225 stated, pull the MC loose from the booster and look at the pushrod, if it's adjustable that is one
option. Does the pedal get really firm when this occurs? Also check the strut link between the pedal & booster, is there a small amount of play, did it go on easy or....?
Also I assume You left enough air-space in the reservoir for the fluid to expand
w/the underhood heat etc. Unless all 4 sets of shoes are adjusted too tight,(not
common but possible), fluid expansion is the only other temp/time related item.