New GM Brake Pressure Differential Valve Rebuild Kit!
I've just added a new rebuild kit for 1967 - 1975 GM brake pressure differential valves to my inventory! I'm still researching the original applications, so please feel free to contact me with questions, corrections, or additional information if my descriptions are incorrect or incomplete. Here's a link to the kit:
1967 - 1975 GM Brake Pressure Differential Valve Rebuild Kit
and a link to a pair of technical articles that describe disassembly: