95 grand am brake problem

IF my car sits for a while and i take off just down the lane then the back brakes lock up as if the ABS was kicking in but after i drive about half a mile they stop doing it. The brake pedal goes down about half way before working also like theres air in the line. the back left wheel cylinder was leaking so i changed that but it still does it and i bleed all the lines. I think it might have something to do wiht the master cylinder but im not sure. IT shakes pretty bad when im giong about 55 and i start to slow down.

Many problems in one system I see. Drum brakes have a tendancy to grab under the right conditions after sitting for a period of time, not to much you can do to fix it except clean them and make sure the drum is round, the pads are clean, and the springs do their thing. When the pedal goes down too far it's an indication that either your drum brakes are out of adjustment or you have a significant amount of air in the system. A sign of air is a spongy pedal. For the shaking is this when you apply the brakes? If so the front rotors may be warped.

To fix the brakes I'd fix that wheel cylinder right because it shouldn't leak, have the rotors and drums turned, replace any worn pads/shoes, and bleed the system starting with the master cylinder first.