in response to a message on the NCRS # 66867
Never use internal lock rings with "ears" on C2 Master Cylinders
Internal locking rings with the 2 ears should never be used the master cylinders of 63 PB and 64 PB models. With their 7/8 ID bore master cylinders the rings internal ears literally scrape a groove in the plastic portion of the PB booster that slides in and out of the master cylinder with every application of the brakes. It is not as troublesome on 1" ID bores unless the ears are prominent.
GM called this plastic assembly (that includes the diaphragm, o-rings, reaction levers and rods, etc.) the power piston. As it slides back and forth it becomes grooved and thus leaks air i.e. it will not hold vacuum. Look in the parts and illustration manual and note the front plastic piece that supports the front rod. On 63 PB this plastic piece is called a plate (group 4.922). On 64-67 it is called a retainer (group 4.934).
I only discovered this problem when rebuilding and replating C2 PBs in the late 1970s. Customers would send in booster and this piece was already grooved and thus a leaker. Even back then, neither GM nor Delco sold either of these 2 pieces anymore. Therefore the groove needed to be filled and carefully sanded down. Even then it was not a perfect solution, because the front seal needs a perfectly round plastic power piston to ensure a good seal. Most often it was better to replace this piece with a used donor.
Although we never had a claim, in 1980 my liability insurance carrier objected and we discontinued this rebuilding service and several others. Prior to stopping these services in the 1970s, a Corvette buddy called, affected a German accent and jokingly asked Is dis herr doktor Rebuild ? and that is how we got that moniker.
PS Someone recently wrote about adjusting the front push rod that is only possible with some 63s that had a screw in the front of the rod. Evidently this was deemed as not necessary since not all 63s have the screw and none of the 64-67s do either. If the push rod does not rest on or in very close proximity of the master cylinder's Aluminum piston, the application of the brakes cause a sudden slap. As the power piston moves forward without encountering the resistance it slaps into the master cylinder's piston.