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.

[JeffYoung]

That sounds like something in the suspension/hub causing knockback, not the booster.

[GKR_17]

Could be a bad wheel.

[chuck baader]

Brake booster...changed everything except the bb and had the same problem. Changed bb...problem solved!!

[awegrzyn]

Your problem above could be everything, but the brake booster. Brake booster has nothing to do with a soft pedal, or pedal going to the floor. Brake booster will cause hard pedal.

JeffYoung's post could be it, but other things could cause the problem. If you really want to isolate technical problems from on track brake pad knock back behavior, all you have to do is jack up the car and brake while running the car in the air. If you can't duplicate the problem look to all the reasons why brake pad knock back exists.

[JeffYoung]

Yeah, brake booster failure shouldn't cause a soft pedal. It doesn't have anything to do with hydraulics.

If it fails, braking effort should go way up, not the pedal getting soft.

Could be the master cylinder, but you changed that right?

It sure sounds like a knock back/hub issue to me.

[chuck baader]

OK...consensus is brake booster. All those who have had a booster fail in exactly that way RAISE YOUR HANDS:026: Go to junk yard, get booster, change...win.

[JeffYoung]

Actually, the consensus is far from brake booster.

Explain to me how no vacuum assist on the master would cause soft pedal?

It COULD be the seal between the master and the booster, but that should have been changed with the master.

Help me out. I'm not understanding how a failing booster cold do this. When mine failed, hard pedal.

[chuck baader]

I attribute this type of failure to magic. Can't explain it but my pedal acted exactly the way Tynor's did. I changed fluid/calipers/pads/MC...everything but the booster. When I changed to booster all was well.

[Dano77]

The booster isn't failing due to leaking, its failing internaly and NOT releasing Vacum. So TOO MUCH BOOST is created. Causing too little pedal feel and less effort. Happens more when the damn thing heats up.

Notice its mostly an RX7 thing. ergo it defies all logic....

[Ed Funk]

RX7.....logic....never seen those words used together before.

[TomL]

I had the opposite problem once, also at Roebling. When I hit the brakes even half-way, all the tires locked up. A couple of RX7 guys said, "It's the brake booster. Replace it." I didn't understand why that would be the case, either. But I did as they suggested, and indeed that cured the problem. Moral of the story - If experienced RX7 guys tell you to replace the brake booster, replace the brake booster. Even if it doesn't make sense. Dano has it right - it's an RX7.

For those of you that say it's pad knockback, you should know that Steve had his brakes working fine for several sessions during the test day. Then the problem appeared suddenly during qualifying. Knockback isn't a problem that is likely to appear suddenly.

[tynor]

Thanks all for the diagnosis. A new brake booster is on order (not too much more expensive reman'd from Autozone than from a junk yard).

I would have never guessed it was the booster - a pretty counterintuitive symptom. But y'all have convinced me it's worth a try!

Thanks again - and hope to see you at the track soon,
Steve

[spawpoet]

Back before we started IT racing our Z, when we were running autocross to sort our car's issues out we had a brake issue we spent months trying to solve. The car would bleed up to a good pedal, but the pedal would go to the floor when it was running and the booster was getting vacuum. Out on the course you would have a super soft pedal that would catch at the very bottom of the pedal, and when it did catch it would lock the brakes hard. Made it almost impossible to autocross. Thinking it was a hydraulic issue we changed everything hydraulic out twice, to no avail. Turned out there was a little rubber disc (called a reaction disc) in the booster that's in the path that actuates the master cylinder that had fallen out of place. Without it the brake actuation was to abrupt, and pedal was extremely soft, so it another way a booster can lead to a soft pedal. I don't believe this is the problem th OP is having, but it can be a headache to diagnose.

[tynor]

Any advice how to "bench test" the booster so I can have some confidence that the new one solves the problem, prior to overcooking turn one at Road Atlanta?

Even with the engine running and pulling good vacuum, I've never felt a problem in the driveway/paddock. Sure would like to be able to somehow validate that the old booster is bad (and new one good) before putting the car back on the track. Possible? All the "booster diagnostics" guidelines I can find are for the "not enough assist" failure mode (i.e. bad = too-hard pedal) -- not this "too much assist" mode.

Steve