This has happened to me before. My pedal wouldn't produce any pressure and it was only the Slave cylinder then later it happened again and this time it was the Master but after both were fixed the problem stopped but I would know for how long because the car was stolen. But I have a better car now anyway. Last car was approaching 150miles before the clutch started giving problems. Occasionally the clutch would start giving clutch problems after master n slave fix but it was at the mileage for a clutch change.