Abstract: A leak detector is connected in the main supply pipe from a gasoline storage tank to a plurality of retail gasoline pumps. A submersible motor-pump unit supplies gasoline through the pipe upon actuation of a nozzle switch at each pump. The detector, at turn-on, monitors the line pressure, and reduces flow if low line pressure indicates a system leak. Premature nozzle opening may simulate a leak and actuate the leak detector. A timed interlock switch in connected in series with the nozzle switch from the main power to related dispenser flow valves. The interlock switch includes a Triac in series with the power supply and main solenoids and a timing device including an integrated circuit having a pulsing network and a D.C. input connected in series with a rectifier to the power supply by the nozzle switch. The interlock switch has a fixed time period during which the respective flow valves are held closed such that an open nozzle does not simulate a leak.