That '86 MC just may not be moving enough fluid. With that said, remember that brake fluid is virtually non compressible and that all you are doing in effect is moving a solid that can go around corners to push something flexible out. I believe that what you have are two opposing wheel cylinders (Lockheed brakes) in the rear similar to what Chrysler used for many years - and were less then great. If you need 3 oz to move the wheel cyls out and are only moving 2 oz ..... You have said that the replacements are in the $90 range - and that others have cornered the market. If indeed these are opposing wheel cylinders, look at the various, at least through the '50 and possibly further if I recall, choose one that is close in size and give it a try - I use Rock Auto Parts for my search.