1989 Chevy Cheyenne Bad Clutch?

When I would come to a stop, my Chevy P/U would still try to move forward, even though the clutch pedal was pressed all the way to the floor. I found a bad seal in the master cylinder & replaced it - this fixed the problem for about 3 months. Last week the problem came back, and now it won't even go into gear. I have replaced the Slave Cylinder and the Master Cylinder & I have tried to bleed the system multiple times. I can hear air, but see very little fluid coming through and have absolutely no tension in the pedal. The pedal returns to the "up" position, but has no tension at all.