I have a GM brake system From a 1975 Olds Cutlass (Main cylinder, front disc brakes and proportioning valve). The rear axle is a 9" Ford and the drum brakes are very inefficient, though everything's new. The rear brakes work, but seem to need more pressure than the GM drum brakes did. I'd like to get the innards out of the prop valve and reconnect the lines. When I open the screw on plug on the rear of the prop valve, where the rear line goes in the center, there's just a round thingy inside with a small hole in it... Does anybody know how to disassemble this thing?