1970 Brake light warning switch...How does it work?

[Tremelune]

I have a 1970 911T. It has a switch on the brake master cylinder that will light up a dash warning under some conditions. How does it work?

Like, how is it supposed to work? My assumption is that, if it has no pressure when the pedal is pushed, it lights up a warning. How does it know pressure isn't just low because I'm not pressing the brakes at all? My only thought is that the positive side comes from another switch that looks at pedal travel or something, but that would involve isolating the handbrake warning circuit, which also lights the dash light regardless of brake pressure or pedal travel...

If I'm not mistaken, the early cars had a hydraulic brake light switch in the master, but that is not the case for a '70 or later.

[jeff eelkema]

Fairly sure your car should have a dual circuit master cylinder that will give you some brakes if you lose pressure from a hose failure or major caliper leak etc. If this happens the front to rear pressure balance will change rapidly and the safety piston in the cylinder will slide to block the leaking circuit and leave you with some brakes and this will trigger the switch to bring on the "brake fail" warning light (just in case you don't notice the soft, low pedal and limited braking). Most cars used this type of system from late 60s but not sure of exact change in 911s.

[savaden]

There is a sliding pin in the master cylinder. The sliding pin has a groove in the middle. The warning switch is not a pressure switch, it has a pin that aligns with the groove in the sliding pin. The ends of the sliding pin are open to each circuit in the master cylinder, front and back. Under normal braking, the pressure from each circuit applies the same pressure to the ends of the sliding pin and it does not move. If one circuit loses pressure, the higher pressure in the other circuit presses on one end of the sliding pin and it moves. This presses on the pin of the warning switch and the light comes on.

One thing of note: If you use the pedal pressure method of brake bleeding, and one circuit has pressure and the other does not, it can set the sliding pin when you are bleeding the brakes. It is then a PITA to get the warning switch to reset.

[Tremelune]

Aha! Very helpful, thanks.

Is there any chance this could light from air in the lines or a stuck piston situation? My brakes seem unbalanced, but I can't see any leaks anywhere.