If the truck came stock with HydroVac brakes, which it sounds as it did, there will be a check valve in the system mounted on the engine bay firewall. Follow the 1/2" vacuum line from the engine up to the firewall, the first thing should be 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
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