i have a 73 k-20, it has always had "soft brakes" I tried to bleed them the other day... now i have only rear brakes.... first i replaced the master cyl still only rears then i repaced the frt calipers and pads still no brakes today i replaced the proportioning valve still only rears....the front have good pressure on the bleeders, they will hit the upper fender wells with enough force to get you in the eyes regardless of where your laying...ha ha...ive gone thrugh a gal of dot 3, and cant get them to stop spitting air..there are no leaks that i can see. im going to replace the rubber lines tomorow.. i dont think their bad but what else can i do? any ideas out there?  

When you bleed it are you pumping up pedal and then holding it while someone opens bleed valve and then not releasing pedal until valve is closed? Also, start bleed with furthest wheel and then work back to closer one on same axle.  

yep. ive followed proper bleeding tech each time no air in the back  

Caliper can be a pain to bleed sometimes when they get a air slug in them. If master cyl is good you need to make sure you keep it full and try again. Pump pedal up several times until firm then bleed and reseal a few times and it should come up.  

Did you bench bleed the master cylinder before installing the new one?  

so heres the deal with the brake... today as i was once again bleeding the system (i put on new front rubber lines) as i was standing next to the truck pissed once again i hear this very faint hiss noise.... after several mins i finally labeld the new master cyl as the culprit.the problem ended up being that the cyl in the master was stuck about an inch in and had finally started leaking into the booster. so i replaced the master with another under warrnty and replaced the booster with a used one.... after bleeding it i am glad to say she'll stop an a dime and leave five cents change.............it just goes to prove dont always assume just because a parts new its not the problem. or cant be bad.....thanks for everyones help, I appreceate it...and yes i did blench bleed it first........