a differential pressure delay valve for vacuum controlled devices is disclosed . the valve has input , output and third chambers with a controlled flow orifice in a separating plate between the input and third chambers ; a diaphragm operator with a controlled flow orifice between the input and third chambers ; a stem with a seal on one end attached to and operated by the diaphragm to open or close a port in the separating plate ; and , a bias spring of any predetermined level to maintain the port in a closed position is located in either the input or third chambers . the diaphragm operator opens the port to balance the pressures in the input and output chambers when the change in input pressure or vacuum both is greater than the bias force of the spring and too sudden for rapid dissipation through the fixed orifices . at pressure changes less than the bias force of the spring the valve operates as a standard delay valve .