The master cylinders used between 76 and 78 had a pressure differential switch built onto the body of the cylinder. The purpose being to warn when either side was lower in pressure (failing). The switch has a single wire coming from it and originally had a small black rubber boot covering the reset button.
To reset just push the button. If the boot is missing the button looks like a 1.5mm square protrusion of about and eight of an inch. If it can't be reset or doesn't stay reset I would suspect a pressure or air problem.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician