swapped a rear GM 8.6 axle with disc brakes from a '99 Sierra into the rear of a '77 Malibu station wagon. Even with the metering valve removed from the Malibus combination valve don't have any stopping power, hard pedal, but slow to stop. From reading various threads on this forum guessing it's that the bore on the old Malibu drum/disc master cylinder isnt big enough and cant drive enough pressure for disc brakes on all four wheels. So go back to junkyard and get the booster and master from the Sierra the axle came from? The Sierra has a single reservoir master but the bore is 1/4" bigger....