About half way through a 40 mile commute, the brake pedel will go nearly to the floor. Pumping doesn't bring it up much but after driving on for 1-2 minutes at 50 MPH the brakes are back to normal. This has happen twice in ten days. No brake failure light and the resevior is full of brake fluid. No aparent leaks anywhere. I did notice today just before it happened, a slight smell of warm rubber or oil or maybe brake fluid. Any ideas appereciated. jp

As others have suggested, brake fluid may be boiling. When did you replace it last? It may have absorbed a lot of water resulting in a lower boiling point.

Drive a short distance and see if any of the wheels are radiating excess heat. This would indicate caliper piston seizing.

Also, solid rotors (non-vented) with an air dam can result in overheating.
--
1980 245 Canadian B21A with SU carb but electronic ignition and M46 trans in Brampton, Ont.

I think a good guess would be that a caliper is binding (or flex line is clogged) enough to boil the brake fluid causing the low pedal--then it releases, the fluid cools and the pedal becomes normal. This is not something to fool with. If you can't do it yourself get to a shop as soon as possible to have the problem properly addressed. -- Dave

You don't mention having done any brake maintenance. Now's the time to do it.

Inspect the pads and calipers for smooth operation -- make sure the pistons retract and the pads do not bind in their ways. Flush the fluid.

Next time you feel the problem with the pedal, interrupt your commute in a safe parking area. Use your palm at a few inches, or an IR thermometer to check each disc. If all this checks out and you still have a mystery, consider a new master cylinder.
--
Art Benstein near Baltimore

Annual Performance Review Time...
"He would argue with a signpost."

I can not understand how you could drive 50 mph after your brake failed. are you crazy?