My clutch is really really soft!! 1989 s 15 pickup

I have a gmc s15. My clutch is so soft that i can push it all the way down and it will no engage the clutch. I have put fluid in the resevoir and replaced the lines. I can pump the clutch pedal for about 20min and it will start to get hard and be the way it should, but then i leave it alone for about 30min to 1hr and it goes soft again. I'm pretty sure there are no leaks. plz plz help got to get to work somehow :)

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