You could test your theory of failed hydro-vac. Disconnect the brake line that comes from the M.C. at the hydro-vac, connect that line to the tee that feeds the rest of the truck. Now you have a manual system and if all is well after bleeding you should have solid braking and no loss of fluid. I'm running my '57 two ton in that manor and the brakes are good even with a full load of hay on it.

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