Hy oldieoldie, to test the Hydrovac you need to have functioning brakes. If you have functioning brakes, with the engine off pump the brake pedal a few times, now place your foot on the brake pedal and press down on it and hold that pressure, now start the engine, if the Hydrovac is functioning properly the brake pedal should move closer to the floor when the engine is started. Hundreds of trucks were driven tens of thousands of miles safely in all conditions with the vacuum source from the engine being shared by the windshield wipers and the Hydrovac. Adding a dual action fuel pump will improve the function of the vacuum wipers, hope that helps.
