Did a brake job on a Sporty years ago and had a similar problem. Turns out the rubber hose to the caliper had collapsed inside and would not allow pressure to the caliper. Some fluid would leak by so I thought M.Cyl was bad so I rebuilt it and same problem, replaced the hose and all was good. Brake fluid had eat up the inner part of the hose.

 

Hi,Thats a new,and interesting, "angle" on what the problem might be. Will keep this in mind when fixing. I have noticed that the front hoses look original (20 years old). Bungo.