Clutch problem--Need help

Took the car out today for the first time this year after some rebuilding over the winter. Having a clutch problem and need some advice from the Apex community. We are trying to get ready for the historical event at Indy in two weeks.



Here is the problem...We bled the clutch and it worked just fine in the paddock, pit lane and onto the track. However, after several laps the clutch pedal went limp. I don't use the clutch when shifting but noticed the limp clutch pedal when I returned to the pit lane.



We proceeded to bleed the clutch again and it worked just fine in the paddock, pit lane and onto the track. Again, after several laps the clutch pedal went limp.



Back to the paddock and we replaced the master cylinder because we had a spare. Same result.



Here are a few other facts...



I replaced the o-rings / seals in the throwout bearing over the winter. Seemed pretty straight forward.



When the clutch pedal goes limp, brake fluid is pushed into the master cylinder until it spews out of the vent hole in the cap making a mess of things. We top off the master cylinder with fluid about .25" from the top. After the pedal goes limp, fluid is at the very top of the reservoir and excess fluid is pushed out the top of the cap.



So, I have at least two questions. First, why is fluid being pushed into the master cylinder?



And, why is the pedal going limp and I'm losing my clutch?



Any help would be very much appreciated as we have only one week to prepare for Indy.



Many thanks in advance.