master and slave cyl.

i been having a problem with my 95 fullsize. the master slave went on me so i replaced it. then about a month later, it went again. i put the 3rd one in it and i didnt do crap. i replaced the slave cly. b/c that was messed up as well. still will not go into gear. seems like alot of air in the line. i bleed and bleed and bleed. worked for a bit then wouldn't go into gear again. bleed more. i cant count how many times i did that, to many, i replaced the line between the 2 and it seemed to work for about 20 mins. i changed the slave again becasue autozone parts are junk. well i kinda blew the slave because the clutch fork was not fitted right. after the new slave (3rd one) it seems to be ok now. but i have a feeling it will happen again. it will not go into gear. seems like air keeps getting in. sometimes when i bleed it there no air. is this the fork or what. it looked fine, but hard to see in there. i dropped the plate, the slave and the started to look, but its all inside. do i have a major problem or what. any help would help me greatly.i know it will happen again. its a 95 1500 series, 1/2 ton 4x4 v6 4.3L speed.