Brake booster rebuild kits.

youtube has a few vids on Bendix booster rebuilds. Seems the hard part is the disassembly of the tabbed versions: some bolt up, some have V-bands. You need to put it under a press, and clock the 2 shells under a good amount of pressure to locate the tabs and then twist-lock them (imagine 2 huge tanks sender ring wrenches on the 2 shells) or set the tabs with a drift. But it looks very simple inside. Chevy used Bendix too so the parts may cross. Seems rebuilders exchange these for fast turnarounds.

here, 67 Bendix rebuild kit...for a Caddy! $117...not much cheaper than a reman?
1967 Cadillac - 1970 Cadillac "Bendix" Power Brake Booster Repair Kit