I found this little article about master cylinders and residual valves. Photos are crappy but good info. http://www.chtopping.com/mustang.html http://www.chtopping.com/example6/example6.html http://www.chtopping.com/example8/example8.html
I also found this about a master cylinder on an older mustang with manual brakes and 4 wheel discs. The used a mc from a Lincoln Mark VII. 1 and 1/8 bore. http://www.ultrastang.com/Page.asp?PageID=18
according to his 3rd paragraph, the switch on the proportioning valve activated the brake lights...? Did old Mustangs illuminate the brake lamps using a brake line pressure switch? Im pretty sure that was to illuminate the red-brake light dash indicator if there was ever a major difference in braking pressures, indicating a problem.
Yeah, I shouldn't post when I'm tired or under the influence of an adult beverage. I was thinking the same thing
I think the ones that had a single reservoir mc cylinder had a pressure switch to operate the brake lamps on the back of the car. I guess he improvised one with a micro switch on the pedal linkage. The switch in the photos on the proportioning valve must be wired to a brake warning lamp.