Hy Iron Clad, the fresh air filter in the cab should never be under "vacuum", you can sometimes here when the booster is functioning air going through the filter to the booster (Hydravac). As Mike B said your Hydravac is leaking from one side of the piston to the other, so my advise is a complete rebuild of the brake system. Incidentally HVAC stands for Heating, Ventilation, and Air conditioning, is that what you're referring to in using those letters?
