Been working on this 83 cutlass and having problem getting enough brake pressure to the rear wheels. Short explanation is it has a 12 bolt rear end from a 69 Chevelle and was converted to disc using PBR single piston calipers. Foot brake drag car and will turn the rear wheels anything above a high idle. Tried different prop valves and finally plumped system without one and replaced all lines. Same problem. Going back to drum on the rear to see if any difference. Got some 11 in backing plates from a 77 Pontiac and would like to use a 7/8 wheel cylinder. Stock is 15/16. Any 7/8 cylinders work in this app? Will 15/16 be ok? Local Parts stores just give me the deer in the headlight look. thanks