Ive replace all brake components twice,this has been ongoin fro 2 years now. ive tried several master cylinders ,blew through all lines with success clearing all... now one thing that im not sure of is the rear flex line ,theres a place for the flex line to mount but this flex line doesnt mount..but its on there and rear brakes never locking up. i have one line from the master cylinder that splits. usually they lock up on the left but its happened on the right also. the immediate remedy for this is to open &bleed(of fluid) the line near the master cylinder and fluid shoots out til the backpressure is relieved the you can travel until you brake about ten mo=re times , then it locks up again.ive mastered driving without brakes until i need them desperatly.and then not driving at all.

 

I could fix it if you were in PA? Anyhow - did you bench bleed the master cylinder? You have new wheel cylinders/calipers? Lockup must be in master cylinder or wheel cylinders unless you have a kink, bend or blockage in a line. I've seen this happen due to junk new master cylinder lids where water gets into the fluid. Your fluid should be clear - not brown. Brownish indicates contamination. Never bleed fluid at the master cylinder connections - damage can occur to it. When bleeding fluid start in the rear passenger (right) side bleeder go to rear driver (left) side bleeder then front right and last front left bleeders.

 

When bleeding brakes master cylinder cap MUST remain on for each bleeding. Bleed until air bubbles are GONE from ejection (easier to see if you use a wrench- not ratchet). Bleeding by a tool could cause damage. Use a buddy if possible and bleed this way: buddy pumps pedal about 20 times to start and holds it to floor DO NOT RELEASEPEDAL UNTIL YOU HAVE OPENED AND RE-CLOSED THE BLEEDER SCREW. Then re-pump, re-hold pedal and re-bleed over and over at right rear until ejection streams out at at least a 45 degree angle with NO air bubbles. This will eliminate a large portion of air. Then re-pump, re-hold pedal and re-bleed rest in order I said. If you have trouble after that it's mechanical unless you have calipers on the front and a proportioning valve which would then be a likely culprit. I know disk brakes were on in the 70's Fords, and that 1964 and up still had front wheel cylinders instead of calipers. If I knew which you had it would be a plus in diagnosing the problem.