Hydrovac is a descriptive name, I believe Bendix made them and yes they are vacuum actuated. Many other brands used them as well like Ford and Dodge.
Turn the head lights on and see if they dim or go out when you hit the starter. If they do then the battery is weak.
Put the truck in High Gear and gently rock the truck back and forth and see if the motor turns. Make sure to have wheel chocks on both sides of the tires (about 6" away so the truck can roll, but not get away from you if the motor breaks free.
Mike B
