the plunger inside the Prop valve that triggers the sensor might be stuck off center. or the internal pressure sensor plunger could be leaking forcing the sensor to trigger off.
What can happen is the sensor had sat happy in the center for 40 years, you pull the master drain the fluid for a brake repair. then refill and either a piece of dirt has jammed the plunger or a seal went, or could even be an air bubble inside the prop valve causing the plunger to sit off center.
you could try a vacuum brake bleeder, but you may have to pull the prop valve out, or attempt to open the bottom nut on the unit to release the plunger and spring mechanics and rebuild the prop valve.
it could quickly turn into a nightmare with original 40 year old brake lines.
today you can get a replacement prop valve or a rebuild kit. 8 years ago neither was available. I had to cobble together 3 junk yard prop valves into one working unit.
i also had to replace ALL the brake lines on my car as they snapped off when trying to remove the prop valve.