Originally Posted by Irelands child
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.
Not moving enough fluid is a possibility, I would not know how to tell though. The original master was if I remember correctly a little dinky thing but I guess in this case maybe size does not make much of a difference.
The wheel cylinders are real strange in how they fit into the backing plate, they are un-like anything I have ever seen. I have poured over Napa catalogs loaned to me from a Napa friend and have found nothing like them.
I guess I could modify anything to work but I have to figure the time it would take to do so.
What do you think about the post made by Poncho, the front reservoir is much smaller and that is the one that fed the disks originally. Also the proportioning valve deal makes sense.