Have you fixed the problem yet?

Was thinking that if the piston is lodged to one side (the side that had the lower pressure from the leak), you could try this:

1. Have someone press on the pedal while you crack open a bleeder screw (eg. top) on the left caliper. The reduced pressure would cause the piston to move to that side. Tighten screw.
2. If that side is already the side the piston is on, then the light will stay on. If it wasn't, the light will go off.
3. If light stays on then, press pedal, crack open other bleeder (either lower ones). Piston should move to other side. Tighten screw.

DON'T let pedal sink all the way to floor, else master cylinder seals may move into unused crudded area and may get damaged.

You may be able to do this alone if you use something (eg. 2X4) to push pedal down (butted against seat) while you crack the screws. Main thing is not to let up on pedal while screw is open else air will get sucked in.

NOTE: Instead of removing the wheel to access the bleed screws, you could try cracking the lines at the master cylinder since the system is closed beyond the MC.
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1980 245 Canadian B21A with SU carb and M46 trans in Brampton, Ont.