This evening when starting the car, the first few times I let out the clutch the pedal did not spring out like it should. The clutch engaged and disengaged fine. It felt to me like there was air in the system. I have just had the transmission fluid changed, and at that time, the mechanic who specializes in transmissions (corvettes, not volvos) checked out the cylinders. The master cylinder was seeping a little, no leak, and he told me it was nothing to worry about. I just got the car about a month ago, and since I got it I have noticed that when I let out the clutch sometimes I would hear a little twang a moment or two later. It sounds to me like the cylinder getting hung up and not expanding fully when the clutch pedal is released, but then breaks free and makes that noise. So I guess my questions are:
Is the master cylinder likely to be the cause of the air in the system that I felt tonight?
How much time do I likely have before the clutch becomes inoperable?
How much does the part cost and how many hours does it take to install it?
Does the slave cylinder need to be changed at the same time?

I know this sort of diagnosis is hard to make on the basis of someone else's but I hope someone has some insight.

Thanks in advance.

AJU