2003 Ford F150 Have to pump clutch pedal occasionaly to shi

I occasionaly have to pump my clutch pedal to feel any pressure and shift. I seems to do it more on hot days or on start up when the A/C is on. It's intermittent, so I have no idea where to start to look for the problem. The fluid in the mastercylinder is clear and full. I have never seen a puddle under the truck that could be hydraulic fluid. Usually 2 or 3 pumps and the clutch will disengage, but it comes and goes.