I have a 1972 C-10 with front disc and rear drum stock brakes. They are manual brakes and everything is new (within the last year) except the combination valve. The problem is that there is no rear brakes at all. There is no pressure at the drum cylinders. The fronts still work though. From what I have read, the combo valve has everything to do with front and rear brake distribution. I figured that i might as well replace it since it is the only original brake part on the truck. One final piece of info, all this happens after it has been parked all winter. Please help if you can. Some new old stock would be nice.