Early Silver Shadows have a master cylinder to provide brake pedal feel.  This includes cars up to approx. xxx22117.  Please check the Spares Catalog for exact fitment if you're not sure.
This master cylinder has a larger 3/4" bore than the original which is 5/8".  This improves the pedal feel. 
This master cylinder is a direct replacement for the original although the threaded stem may need trimmed.
Don't bother rebuilding your stock unit when a brand new one is $65.