In my 68 Chevelle the passenger side front drum brake keeps locking, it will not release pressure. The only way to get it to come down is to let loose that nut behind the brake to let the fluid bleed out, but that doesn't solve the problem and it puts air in the system.

Any ideas on what could be causing this/how to fix it?

Most likely a blockage in the rubber hose between the wheel and the frame. Deteriorated rubber can act like a check valve blocking the return flow.

Hope this helps.

Rob

It's a flex line problem. Replace it and bleed it and off you go.