Power Brake Master Cylinder

Production master cylinders for 1969-1970 Mustang/Shelby with power disc brakes are identified by a dimple near the front outlet (1), Bendix logo and 2227161 was cast into the bottom (2) and a dome hex head bolt (piston stop) on the bottom (3) The master cylinder, brake booster and cap were Painted semi-gloss black as a whole unit. Plugs were used to keep spray paint from the threaded outlets so you will see a round bare steel circle around each exit.

In the upper picture (original 7K mile Shelby) you can see that it didn't take much brake fluid to remove the light coat of black paint on the master cylinder itself although a white paint dab did survive.

Boss 429 only notes: Boss 429 cars had a unique master cylinder with a 15/16" piston bore, larger outlet ports (9/16" front / 1/2" rear), hex headed stop screw, and a 2227091 cast on the bottom. They also had a used unique brake pedal and support identified by a spray painted orange dot.

Why are they different from the standard Mustang disc brake master cylinder?
The bore size was reduced to create the required fluid pressure with the unique BOSS 429 brake booster and pedal geometry.