It distributes the braking force when the bed is loaded down. With the bed off, it could be directing all the fluid to the front brakes. You could try setting some weight on the frame to lower it and see if it bleeds. They do get plugged up from non-use so a junkyard replacement isn't advised. Plan B would be to just bypass it.
Plan B wouldn't be a smart move. Rear wheel lockup with no load isn't quite the ideal situation when braking hard.......................
