I have a 1988 FC that seems to run pretty well for a car of its vintage. But today after some driving (and sitting in the North Carolina sun!), I noticed that the brake was pressing further and further down. By the time I got home, the pedal was going almost to the floor.

Brake fluid level seems fine (reservoir is up to max), and I don't see any leakage around the master cylinder.

Any thoughts about what could be going wrong? After about an hour or so in my garage I took it out again and it was fine.

 

you master could be leaking internally.

 

Thanks for the suggestion, misterstyx69. It turns out one of my calipers is dragging and my brakes are overheating.

I'm going to replace the caliper. I could try cleaning the piston, but a new caliper is cheap enough.