Vehicle: 1994 Civic with no ABS

Problem: I am having a weird problem with my brakes. A few days ago, on a
stinking hot day, I was at a set of traffic lights, when suddenly, my brake
pedal began to slowly sink to the floor. The brakes worked fine and would
stop the car, even under severe braking. The only problem would be that the
brakes, when depressed would begin to sink to the floor after a few seconds.

They were fine in the morning and fine in the evening. The only difference
was that when I drove the car during the day, the weather was over 35
degrees C. This problem occurred twice and each time it occurred, it was on
a hot day. As the day became cooler or when the car is driven on a day
below 35 degrees C, the brakes behave normally?

Any ideas?