Trap primer operates by sensing the pressure drop in the supply line from downstream fixtures or appliances. The pressure reduction above the valve disc causes the disk to move upward, opening the lower disc seal and closing off the upper disc seal. With the lower disc seal open, the stored air pressure in the air chamber pushes a portion of the contained volume of water from the lower chamber out through the opening beneath the lower disc seal to prime the trap. When there is no longer a demand in the supply line and the full line pressure is restored, the valve disc lowers – opening the upper disc seal, allowing flow through the one way seal, seating the lower disc seal, and recharging the air chamber for the next pressure drop cycle.

Durable rubber construction keeps the connection between rubber hoses and accessories tight and leak-resistant.