what would make the rear brakes lockup while brakeing

checked that already also the brake light and a b s lights are on also tried bleeding front and rear so could it be most likely be the master cyl or proportioning valve

Rare for a proportioning valve to fail. Almost never. Could be a master cylinder but Im not convinsed of that. If it has drum type brakes the dia. of drums should be checked.

thank you thats what I wanted to know