Manual brake setups use a "pushrod retainer" (basically a 1/4" piece of small vacuum hose), that fits on a groove machined at the end of the pushrod. The master cylinder has a corresponding groove in the back of the piston, and evidently not all m/c's have this groove. I guess the pushrod can fall out if the brake pedal is pulled back too far. I can't remember if my power brake pushrods are lacking this groove, but that may explain the different m/c applications.
