Hy Old GMC Gold, the atmosphere valve is supposed to be activated by hydraulic pressure created by the master cylinder, the opening of the valve allows air under atmospheric pressure to enter the assist side of the bulkhead/diaphragm. The air under atmospheric pressure assists in applying brakes, the more you apply the brake pedal the more atmospheric air is allowed into the assist side of the Hydrovac. If the atmosphere valve is off its seat for any reason other than the way it was designed to be problems with too much assist or brakes applying when the engine is started can occur. If the fresh air (atmospheric) line or filter were plugged little or no assist would be felt, hope that helps.
