2004 Chevy Silverado clutch problems after replacement...... HELP!!!

The clutch and slave cylinder have been replaced. After bleeding the clutch several times and several ways the pedal will only hold firm for a few minutes. Then, it is back to a very soft pedal and resistance going into gear. Any ideas??  

just curious.. do you have the OLD slave cylinder still.....

do you have an OLD valve spring laying around...   some flat washers and a nut and bolt..     you could use a 1/2 bolt.. does not have to fit the bore.. just not pull through the holes in the washers..  there are iron washers at some hardware stores.. if digging in your used fasteners box.. try some strut rod bushing washers..

bolt. big washer. slave cylinder.. washer, spring, washer nut..

this will allow you to connect the OLD slave to your new master to check the ability of the new master to operate each time... without pulling air in through the internal seals...

please.. bleed a bunch of fluid through it before closing the bleeder screw.. so you don't introduce debris backwards into your new clutch master..

and you can do it without pulling the transmission..

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i have another question...   was the clutch master in a sealed bag.. or did it look like it had been returned ...

i have run into parts.. where people have swapped out the new internal parts for used parts and returned them for full price..   they usually do this with alternators.   look inside and the alternator diodes are OLD.. not even cleaned..