Ford: F350 clutch problem, return spring, slave cylinder

Question
QUESTION: 2001 F350 dually 7.3 D. Some times when the clutch is depressed it will lock or push hard and as soon as you release pressure it goes easy. It feels like a hyd. lock and not a mechanical blk. If the master was allowing air the clutch would press easy to the floor. Got me stumped.

ANSWER: have you flush out the fluid.and bleed the system?

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QUESTION: yes we put a new master cylinder on and bleed it and its still doing it.

Answer
Look and let me know if your slave cylinder is on the side or in the transmission. if it is inside you may be fighting a bad return spring  on the slave clyender.or the front cone may be crack or have a grove in it,you can see this by looking through the inspection hole .