Mystery brake failure

[tynor]

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.

[Greg Amy]

Vac brake booster. Try plugging the vac line and see if the behavior changes.

[Flyinglizard]

Check wheel bearings also.

[Dano77]

Brake Booster is failing. Greg had the opportunity to verify my findings at a race last season.

Plug/remove the vacuum line to the booster. the pedal will be hard as a rock, like pushing on the building, but the car will stop and verify the master and lines.

Dan
77 IT7

[spawpoet]

I've gotten a super soft, overshooting the Sebring hairpin down the escape road pedal unexpectedly from pad knock back, but you normally get a good pedal back by the next turn or two. You say you have a soft pedal but with useable brakes in the turns that follow so I'd guess that's not the issue, but worth considering.

[darthmonkeyIT]

This sounds like a similar symptom to my CRX brake booster failure. I would do as others have said and check the brakes with the booster disconnected.

Recomendation for pad kick back - When ever I am near the end of a fairly long no braking section (straights) I bring the left foot over and tap the brakes just enough to engage the pads. This takes out the pad kick back and makes sure you have brakes for the upcoming heavy braking zone. I try to aim for about 4-5 seconds before I am at my braking point so I have time to get my left foot back to operate the clutch.

Does this bleed of a mph off my top speed - ya probably. But with Honda brakes it was often a coin flip if the car would brake "normally" or not. Since I started doing the brake check I have yet to have my brakes not perform as expected.