I'm having a similar issue on my 72. I've been wrestling with the brakes for a few weeks. The rear works fine and I can bleed those lines without issue. However, I don't get fluid flow to the front calipers. Rather, I had fluid flow to the front calipers and no longer do. I have tried two new master cylinders so I know that can't be the issue. The next most likely culprit will be this switch device. Mine is "original" (as in it was in the car when I towed it home 4 years ago) and is missing the electrical sensor. I installed the part during my reassembly and had reasonable braking performance for a few days. Then, I tried re-bleeding the brakes with a vacuum bleeding kit and haven't had front brakes ever since. I now suspect the pressure switch is pushed/stuck to one side and not allowing brake fluid to the front circuit. Does that sound reasonable?

Has anyone removed this part and used something else in its place?

The car is almost completely reassembled and finished but I can't drive it until I solve this problem!

 

why don't you rebuild it, instead of replacing it?

I plan to remove it from the car today and then disassemble/clean/assess. I am still missing the probe that connects to the electrical circuit to activate the light on the dash and I'm unclear about how that will affect the performance of the "plunger" inside that moves when a pressure imbalance is detected. Without the probe associated with the circuit, can the plunger move too far and jam to one side or the other?

Possibly, I believe the end of the sensor acts as a stop

I doubt it's caused by the brake switch. It could be a blockage in the piping to the front brakes.

In looking at the FSM drawing I posted back when I created this topic, it appears that your piston only moves a few millimeters in response to a pressure differential that will overcome the opposing spring force.  The "full diameter" part of the piston then makes electrical contact with the sensor and the warning light goes off on the speedometer.  Once you release the brake pedal, the spring forces "should" return the piston to its central position, breaking the contact and the light will go off.

Without the sensor, it does not look to me as though the piston can move so far as to shut off either brake circuit.  So, I agree with EuroDat and would look for something else that is clogging the system, most likely on the inlet flare fitting of the front brake circuit or in the line coming from the MC..