oldieoldie, if your truck was equipped with a Hydro-Vac as original, it should already have a check valve on the firewall in the engine compartment. I added a 30 lb. disposable refrigerant canister to my '57 when I installed the 2 speed rear. The vacuum reservoir needs to be open to vacuum in both directions and the original check valve will keep the whole piping system under vacuum as long as the check holds. The tank is just tee'd into the existing vacuum line after the check valve.

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