1975 Plymouth duster brake relay question?

Here's how the wire to the brake prop works. That is part of the differential pressure piston in case one end of the system springs a leak,the difference in pressure will push the piston to one end, and that actuates the switch to ground. That will light the "BRAKE" lamp on the dash. In parallel with that switch is the switch on the parking brake which functions same way. When you pull the handle, the switch grounds--just like a courtesy lamp door switch, and lights the BRAKE lamp.