The valve he is referring to is called a "residual pressure valve" used on drums. It has nothing to do with the problem. You should NOT use a residual valve on discs, but it will not stop them from applying. It WILL stop them from releasing. As others have said, you either have something wrong with the master, or it is not bled, or a prop/ dist valve is effe'd up
Or the caliper pistons are rusted "in place"
