Vacuum Checking Relay
This unit performs two functions. In upper part of relay is a conventional rubber check valve which is opened by engine vacuum applied at port "5". See Fig 1 This allows port "3" which supplies vacuum to the rotary valve, to be evacuated. If engine vacuum drops, the check valve closes, keeping the vacuum level steady by port "3".
Lower port of relay is spring-loaded diaphragm which blocks or passes regulated vacuum from the transducer (port "2") to the vacuum motor (port "1"). When engine vacuum is high in port "5" normal vacuum passes to the motor. When vacuum drops the diaphragm blocks the passage and holds the vacuum level at the motor.