I race an IT7, and at the end of last season I had a problem where the brake pedal would go to the floor or very nearly at the end of the long straights. Pedal was ok through the minor braking zones - maybe not completely normal though.
Pumping the pedal brought it back, but I would say the car was just barely ok to drive on track.
My theory was the pads were getting knocked back from the discs in the corners due to loose wheels bearings. I checked the front calipers and brackets, pads, wheels bearings, etc - everything seemed perfectly fine. Bled the brakes, and I tightened the wheel bearings, but I don't think they were very loose at all.
Same situation out back - all appears to be normal.
You can stand on the pedal with the car off, and it doesn't sink at all - pedal is very firm.
I just pulled the car out of storage for this season. I went over the front brakes and bearings again - if there's something wrong, I am not seeing it.
I did notice that the rubber line from the braker booster hard pipe to the engine was badly deteriorated and spongy (sits right above the header). If it's leaking I wouldn't think it would cause the pedal to sink.
Any idea's on what might be going on? I need a fix before next weekend!
Maybe the fluid needs to be replaced because it has absorbed some moisture. So this causes the fluid to boil easily and forms gas bubbles in the fluid after use in the minor braking zones? Bleeding the brakes throughly to replace the fluid may fix it.
I've bled the system 3 or 4 times since I started having the problem, so I don't think that's the issue, but maybe I need to review that a bit.
I'm thinking that pedal to floor only on track is either mechanical slop somewhere or boiled fluid.
I don't think a bad brake booster or overheated pads would give that symptom, right?
Bad booster would give exactly that problem, actually 
You're losing vacuum assist. I would replace the line and the booster (they're cheap).
Huh, I thought the booster just added force to the mech action that you were creating with the pedal.
I thought no vacuum, no extra force equals very hard to actuate brakes but the same pedal travel.
How does one test a brake booster? Can it be done on the car?
That's right, a bad booster increases pedal effort but doesn't change the travel. Sounds to me like you either have the aforementioned 'knockback' (warped rotors can cause that, does the steering wheel or the whole car shake under hard braking, both front and rear rotor warp can cause this), the master cylinder has incorrect free play (is it a new cylinder? If so, it may require adjustment of the pushrod) or the master cylinder has a 'cup' that's almost worn out. The 'cups' are suppsed to touch the bore at all times, if one is worn it will allow fluid leakage until the pedal is stroked, this forces the cup into contact with the bore again. I personally think it's #3. Sometimes in its early stages you can see the cup problem as follows: remove the master cylinder cap, take the filter out, then have a bud work the brake pedal. It's normal to see a small amount of fluid movement as the pedal is stroked but if it looks like fluid is squirting upwards, there's your culprit.
Cool, thanks for the info.
I don't have vibration under braking, but will test the runout of the rotors tonight.
I've got a new MC on order - hopefully that's my issue.
Booster is easy to check.
With the engine off,pump the pedal several times to exhaist any vacuum. The pedal should be hard. With foot still holding pressure, start the engine. The pedal should sink slightly. Still holding pressure, turn off the engine.
In roughly 30 secs. the pedal should return.
