I have read many conflicting reports and articles about proportioning valves and check valves. I have 1970 chevelle that is a power front disc /rear drum brake setup. I am changing to all wheel power disc and i believe you have a master cylinder that has a built in propotioning system . My master is junk anyway and needs to be replaced. with the stock masters they tell you to use a propotioning valve with front disc and rear drums but there are no recommendations for four wheel disc's.