clutch problems.... :(

[Adam_11]

ok so on my way to class last week my clutch pedal went dead... i had no pressure and the clutch wouldn't disengage and i couldn't shift. So i took it to my mechanic and he replaced the slave and master cylinder... So i went to pick it up today and we tried to move it to make sure everything works and the pedal feels the same so we tried to re-bleed it and we get pressure for about 20 seconds and then it all goes away!? im not sure why its doing this any help would be greatly appreciated. 91 240sx SE...