brakes locking on 78 c20

[bns454]

Hi i have an issue with my brakes locking on sometimes.
when it happened today i unbolted the master cylinder from the booster to check if the cylinder was jammed it wasent.
i then cracked the front and rear lines at the cylinder both sprayed fluid out and the brakes released
the brakes then worked as normal.
I am thinking i may have a problem with the valve that all lines go into mounted to the frame below the radiator? What is the purpose of this valve? Should i open it up and have a look? Or could it be somthing else causing my problem