brakes. i think

i have an 84 monte with braking problems they catch on their own then let go. They don't stop the car it just slows it down then lets go only to do it again about every 30 yards or so. I've check both rear wheel cylinders replaced line from the "T" to the rear passenger to fix a leak spliced along the body to fix a leak replaced the hose on the driver front because it was squishing with pedal presure, the new hose fixed it. replaced the passenger front caliper with a new one did not replace driver side caliper because piston travel was the same as the new one all the wheels bled off perfectly strong streams till pedal bottoms. i have no idea what to do is there some kind of switch or valve i don't know about. please help me out thank you in advance