[sideshow]

if master cylinder has two pistons maybe the second is getting stuck
if they have 2 pistons normally closest one to firewall does fronts and furthese does rear

if its single piston then if that sticks u will have alot of freeplay in pushrod so thats easy to tell

when rear brakes are locked have u tried cracking the front bleeder on the mc
if this releases rear brakes it then points more towards master cylinder holding pressure
and not the lines or rear calipers

did it just happen out of the blue
or after u did a pad change