I just converted my 1972 blazer to 4 wheel disc brakes. My big questions are this: I tubbed a corvette with a D60 in the rear and put drums on it. when i did this i had to change the proportioning valve out to a 70's style valve for front disc rear drum. It worked great. Why if i changed my blazer to 4 wheel disc brakes would i not have to change it to like a corvette valve for 4 wheel discs? Are the 2 different resevoirs in mastercylinder completely seperate? In other words could i not even put rear breaks in at all, fill the front resevoir and have full working front brakes? can any air get from the front lines to the rear lines or vice versa?