I have a mystery brake problem. At the last race, my brakes suddenly got spongy. On first session, all was fine - normal hard brake pedal. I took my normal cool down and didn't do anything that should have overheated the brakes; didn't adjust the proportioning valve; didn't touch the braking system in the paddock.

However: At very beginning of second session, on the first hot lap, the pedal practically went to the floor on the first hard braking. I'd not noticed any particular problem on the warm up lap.

I bled the brakes twice and got no bubbles. In the paddock, the pedal feels fairly normal. Reasonably hard pedal and I can hold it indefinitely without it sinking to the floor. So I've concluded that the master cylinder seals are OK. Even with the engine running, the pedal feels normal (so not a vacuum leak - though even if it were, it wouldn't explain spongy/soft/excessive travel - just the opposite).

I see no fluid leakage. Not at the calipers; not at the master cylinder or proportioning valve.

So I went out on one more practice session - again, the pedal felt fine until the first hot lap, then _large_ amount of pedal travel (almost to the floor). I found that if I pump the brakes ones or twice on the straight, I had a spongy, but useable pedal in the turns.

My plan is to do a complete flush of the fluid in hopes that there is air somewhere in there that is not bleeding out with a normal bleed.

But that seems like a low-probability fix. I'm hoping there might be something else I haven't thought of. Ideas?

The car is an '83 first-gen RX7 (IT7). It has good air ducting and I haven't had a brake overheating problem with this car in years.