Triumph Repair: TR6 1972 Injection: Brake warning light constantly on in cockpit, PDWA, brakes

Question
Hello !

Just bought a '72 TR6 and everything's fine, but I'm worried about something:

On the cockpit board, there are 2 lights: Oil and Brake. The problem is that the Brake light is on. It turned off once while i was driving, and i wasn't doing anything special.

What do I have to check?

Thanks in advance !
Michel

Answer
Under the hood, after the master cylinder, and just where the brake lines begin to split off, is a brass 4 way connector with an electric connector in the middle.  This is your Pressure Differential Warning Actuator.  Its purpose was to tell you, on dual circuit brake systems like the TR6, that one side of the brakes (front or rear) has failed.  If you still have brakes, you're OK.  Just realize you may not have your full braking power available.

Inside this, there is a metal "shuttle", such that if one side of your brakes has a pressure loss, the shuttle is forced to the "low pressure" side by the "high pressure" side, and this trips the electrical switch, grounding it out, to turn on the dash light.  Hence the name Pressure Differential WA.

So your options of what happened/what is wrong is:

1) electric short, and the wire is grounding out somewhere else, turning on the light, and its unrelated to the PDWA
2) one side of your brake system has a leak and the shuttle cock moved
3) when pads or shoes were replaced, the pressure from expanding the calipers/shoes forced pressure back up the line, causing the shuttle to move and ground out
4) When changing fluid/bleeding the brakes, the side with the air bubble was lower pressure and the shuttle moved.

I have found I had to grind a bolt such that it matches the profile of the groove in the shuttle, and use that when doing brake work, so that I don't cause the shuttle to move.    Maybe my PDWA was defective or mis-rebuilt by a PO, but was easily "tripped", and my only option was this special bolt to hold the shuttle in place until the system was purged correctly.  A royal PITA.

Find which of the above caused it, fix it, and then reset the shuttle cock, and you should be OK.