From your description, it sounds like the brake shoes are not backing off after using them for a little while. This is usually a heat related problem. There are one of two causes that I'm aware of for heat related problems. Both are the result of the lines or equipment being too close to a heat source.
Since the master cylinder is mounted on the frame, it's unlikely that there is a heat source close enough to it to cause a problem. Also it has a fairly thick body so it would take a significant heat source to cause the fluid to heat up.
OTOH, a line could be misrouted and be too close to an exhaust pipe.
The other source of heat is brakes. If, as Tim points out, the brakes are adjusted too close to the drums so that they're dragging, they will build up heat over time. That heat buildup could cause the hydraulic fluid to heat up, and in a closed system (which is what a brake system is) that will cause pressure buildup.
The brakes should be adjusted until the shoes touch the drums enough to cause them to drag slightly. Then they should be backed off about 10 clicks. As they wear, they have to be readjusted.
When you're adjusting them, you should tap on the drum with a hammer to cause the brake shoes to equalize inside the drum so that the clearance is the same for all 360 degrees.
