A liquid flow monitoring device for monitoring the supply of liquid to a spray nozzle including a deflectable check valve in a liquid supply passage to the spray nozzle, a light sensing device having a light emitter and a light receiver disposed on opposite sides of the check valve, the check valve being deflectable into at least partially obstructing relation to a light beam directed from the light emitter to the light receiver as an incident to liquid flow through the monitoring device to the spray nozzle, and a control system for receiving signals from the sensing device.