Soft clutch some of the time?

Once in a while my clutch pedal gets soft and makes it hard(Impossible) to shift into first. If I let off the clutch and re engage it works fine. This usually happens when I'm at a red light of course. I changed the slave cylinder when I put a new clutch and plate in when I had the engine out a few weeks ago. I bled the slave and have good pedal pressure except the rare occasion that it is soft. I am thinking that the master may be bypassing fluid when it is being held for a while like at a stop light. I don't see any fluid draining inside on the floor and it seems to be holding fluid in the master cylinder. Any ideas?



93 wrangler 2.5