This just came up yesterday, and I read about this elsewhere, so no points for originality, but here's what I did last night to extract a stuck piston from a master cylinder. You can do the same thing to a caliper by cramming the grease gun onto an open bleed valve and plugging the banjo inlet, but for master cylinders that don't have bleed nipples, this can save the day.
Grab yourself a banjo bolt that will fit the outlet of the offending MC, and a zerk fitting, which you can obtain from your FLAPS. You will need a tap set that can thread for the size of the zerk too. You don't really NEED it to be a banjo bolt, any bolt that fits will do, but you'll sharpen less drill bits if it is a banjo.
Drill out the head. I found this a tad easier from the inside out. You will likely have to work your way up in size. I was shooting for a skosh over 5mm to tap for a 6mm zerk, which took me drilling in 4 steps.
You'll have to reverse the bolt to drill the last 2 passes, it will be too large for the passage of the banjo.
Tap it for the zerk you have:
Weld up the hole(s), or you could likely plug it with JBWeld too. We used a spot welder, ZOT, done. Insert the zerk.
Presto, a "Lil Persuader".
Thread the Persuader into the offending MC, pop the grease gun onto the Zerk, pump it 10-12 times and slowly, and gently, out comes the reluctant piston. Hasn't failed me yet!
