So many things can go wrong with brake systems. We don't change the fluids, unless they leak, and the maintenance on them, is shining the chrome. Now for the list of things to check. First look at your fluid, if it is not clear, dump it. All of it. Start out by disassembling the mc, and make sure the plunger is clean and the piston rubber is plyable and not stiff. Blow air through the two supply ports for the fluid, and run a small tip cleaner in them, then air and more cleaner and air. If you have a small brake hone, hit the bore a few wacks, just for shits and giggles. Now, go to the lines. Clean with brake cleaner, and blow out all the crap. Make sure you can blow both directions, and that the amount of air coming out, is equal(as equal as you can determine with your hand, no guage needed here). the rubber line, over the years, can breakdown and create a blockage. Sometimes it is just in one direction, but sometimes, both. It sounds like you have been in the caliper, but the same is needed. A kit, some cleaner(blow throuth the bleeder port, and bleeder, to make sure they too are clear. Fill the caliper, with the puck fully pushed in, fill the lines, and attach to the caliper. Hand bleed the mc, until you get pressure against your finger, and attach the line. Pump the handle, and watch the final bubbles of air come out the mc. Also, as mentioned above, the mc has to be matched to the caliper. Most banannas have 5/8ths mc's, unless they are duals, and they would need a 3/4ers. Hope this helps.