brake problem

ok guys listen.. i got a 1988 gmc sierra 1500 with a 4.3... now my brakes are fucked the pedal goes right to the gorund and dosnt stop for shit.. this happened before and it was the master cylinder i went to the scrap yeard i got a new one... 3 months later the same problem just worse... now i said fuck the scrap yeard and i'm gunna go to canadian tire for a re-built one.. but first i need to no for sure if its the master cylinder.. ok now theres no leaks and the break fluid is always full so its not a line.. and if i pump the brake pedal it gets harder but not alot just enough to stop .. got any ideas it must be the cylinder or air in the lines i donno but i bleed them when i changed it? thanks guys