Have you checked with your local NAPA? They should be able to send it out or take it in as a core for a rebuilt unit.
I'm not sure if White Post does big truck boosters, make sure to inquire before sending it off...
Mike B
I talked to Whitepost about rebuilding the Bendix Hydro-Vac for my '57, they told me they have parts for everything and could do mine. After disassembling mine I chose to rebuild it myself.
1957 Chevrolet 5700 LCF 283 SM420 2 speed rear, 1955 IH 300U T/A, 1978 Corvette 350 auto, 1978 Yamaha DT175, 1999 Harley Davidson Softail Fat Boy