I tried searching to no avail and would like to see if any of the gurus here could chime in.

I started running the Carbotech XP10/XP8 combo for this season (HPDE at NJMP Lightning & Thunderbolt, plus a couple days at Pocono North) and have already done two days on Lightning. I'm also using a fresh, dedicated set of Centric rotors for the Carbotechs along with ATE Blue/Typ200 that gets flushed out before every event.

On the track, the brakes are great. I always take it easy on the first couple of laps to let everything come up to temp, as well as a good cool-down lap plus some driving around the paddock (or at the very least a couple minutes of driving around the paddock if I can't manage a cool-down lap). The brakes never fade in any way.

The strange issue lies in between sessions: when I get back into the car before the next session, the brake pedal is VERY soft. The car will still has a good amount of braking power, just with a really crappy pedal feel. The issue gets weirder in that it actually improves: once I'm on track after a good warm up lap, the pedal gets much firmer as the brakes warm up (pretty much back to normal). From there on, the brakes are golden for the entire session: tons of power, no fade, no soft pedal. I tracked the car last year with OEM pads and didn't experience this, it coincides with my usage of Carbotech pads on the track.

Back into the pits, the car cools down, and it's the same for the next session: soft pedal, brakes warm up, everything is great.

When I bleed the brakes before the next track day, I usually get one good air bubble out of each caliper, about a third the size of a grain of rice. I also notice a slight amount of fluid that escapes past the bleeder threads and runs down the caliper as I'm bleeding the brakes. During the track day and my driving until the next bleed, the level of brake fluid in the resivoir doesn't drop, so I doubt there's a leak anywhere.

Any insight you guys could provide would be great.

 

I used to notice that in my olde race car SpecE30

I believe its Normal

 

Try another brake fluid. In my one-time experience w/ ATE SuperBlue, I got a progressively softer pedal. I've gotten better performance out of off-the-shelf DOT4 from Valvoline, Castrol, and Pennzoil.

 

^I agree. I had something similar happen to me this past weekend.

 

Wondering if it's an issue between mc and booster.... Recall BW chasing this a number of years back.

 

Wouldn't his situation be the other way around tho if it were the brake fluid?

 

The fluid shouldn't become compressible on the track or between sessions in the pits.

Incidentally, I experienced the same phenomenon *way* back when I first started tracking my Datsun. The pedal was fine on track, but got soft in the pits. Theories abounded, including: On track the fluid is being pumped regularly, giving some amount of "circulation", in the pits the fluid in the caliper doesn't move and gets too hot.
Then last run of the day and, surprise, the pedal went to the floor at a critical moment...

Anyway, there is no doubt that the problem I had was inadequate brake fluid. I didn't know any better, it was the same fluid that was in the car when I bought it!

 

I think your brake fluid reach it boiling point. The best way to avoid it is bleed your brake between session.

 

Based on a conversation I just had, I think I will see what swapping in a new master cylinder will do for the car. As something I probably should have stated in the first post, a friend of mine also has an S2000 and the biggest difference I notice between our cars is that is brake pedal is significantly firmer that mine, and that's even after a fresh brake bleed on my car using the gravity method.

 

I was just going to suggest this. You probably have a master cylinder seal that is going bad.