72 Brake Problems

[RB72]

I cant figure out what is wrong with my brakes on my 72 K10.... I have replaced the rear wheel cylinders and break pads and it seems like it is getting air in the lines somewhere, but all of my brake fluid is still in it and nothing is leaking out... If I bleed the brakes then they stop good for the first drive then the brake light comes back on and it will hardly stop..... should I try bleeding the master cylinder or what? Thanks in advance!