My experiance was in 1985. (1968 SS w/ discs)
Going to a stop and ruptured the dry rotted flex line on the right front disc,,,Pedal went soft and car was sluggish to stop.
Idiot brake light lit-up like a xmas tree.
Pull over to investigate, found fluid leak.
The 'loss' of pressure in (The front hydraulic brake circuit) in relation to the intact rear circuit, pushes (axially) a tapered piston inside the distribution block towards the offending (failed) circuits side. The cone tapered center section of the piston contacts a insulated pin (W/ corresponding Brown wire with boot from your front harness) that effectively completes a ground source for the BRAKE light on the dash, there by iluminating the Brake light. I think this electrical circuit is in parrallel with the foot operated parking brake switch also.
Drained front circuit (Hyd), Purchased New Hose, installed, and removed the switch from the dist block and used a small screw driver to re-center the piston and installed the switch and wire, topped the fluid and blead the system and was back in control of braking.
I belive this system is described nicely in the service manuals also.