1994 Ford F-150 clutch problem

Hi guys, I got a 1994 f150 302 5 speed from my father in law...he got new truck about 4 years ago so about 3 years ago he just parked this one with nothing wrong just didn't need to drive it...well 3 years later I get it and the truck will move but clutch seems to have issues...while taking off first gear seems kind of ok but after second it wants to slip on acceleration, I can't smell it burning or anything and it has good fluid in reservoir...this clutch was put in just over 4 years ago and had no problems..I bleed the clutch by vaccum and gravity and it got worse with no signs of leaking..should I take the master cylinder completely off and bench bleed it? Anything you guys can offer is greatly appreciated thanks