In a system for delivering air from an air pump to an exhaust system and accessories for an internal combustion engine, an air flow control unit has a high pressure relief valve, an outlet open to the exhaust system, a vent open to the atmosphere and an outlet open to the accessories with a check valve therein. A valve member is movable in response to an engine signal between a first position closing the exhaust system outlet and a second position closing the vent; and a low pressure relief valve is included in the vent to maintain a minimum low pressure in the unit when the valve member is in its first position.