Manual brakes came on 78, 79, and maybe 1980 GM "G-bodies". It had a stamped steel plate to cover the hole, a push rod retention cup, a push rod that had a special head on it to be retained inside the push rod retention cup so it would not fall away from the back of the master cylinder, and usually a different brake pedal with a smaller foot pad. They came with a 7/8" bore master cylinder from the factory. The master cylinder looked identical to the power boosted master cylinder which has a 24mm bore size. The master cylinders did not have a hole in the back of the piston to retain the push rod, so the special push rod was used along with a special push rod retention cup.