Hy Guys, the Hydrovac functions by using vacuum created by the engine running and hydraulic pressure created by the master cylinder when you step on the brake pedal. The assist is created when a hydraulically actuated valve closes off vacuum to the assist side of the bulkhead/diaphragm while simultaneously allowing air under atmospheric pressure to enter the assist side of the bulkhead/diaphragm. In the brake application situation you should have a vacuum (negative pressure)on one side of the bulkhead/diaphragm and air under atmospheric pressure (positive pressure) on the other side. This positive pressure easily overcomes negative pressure and assists in applying hydraulic pressure to the brakes. We seem to have a situation with this particular brake system where as soon as engine vacuum is created the brakes at least partially apply, which would indicate a problem with the Hydrovac, I would call the shop where you purchased the Hydrovac and explain the situation to them and see if they can offer any suggestions. Your brake system did function correctly for a period of time after you finished overhauling it didn't it?
