Purge starts getting a freqency sig after the engine is warm. It varies in speed . You can feel it pulsing in your hand. They pulse it so the engine does not get a large gulp of air .. it is a controlled on/off vacuum.
Sometimes the valve inside gets gummed up and sticks open/closed. It can be cleaned by squirting WD in the outside port hose and letting the engine vac suck it through the valve.