Found under the following brands:
Grip Rite, Master Force, Rol-Air, Norgren, Snap-On
This Pilot Valve has an Unloader port included, which vents the air when the unit reaches max PSI and "unloads" the pump, so that the air vents into the atmosphere. The knurled knob on top is turned to allow the unit "dual control" operation, which allows the unit to run in "stop/start" mode or "constant-run" mode. NOTE: Please notate if this is used on a constant-run system with head unloaders or not, when ordered.
If you're a contractor and put heavy demands on your electric air compressor, consider the "187" Dual Control Kit!
A "dual control" kit modifies your air compressor so it can operate in either "stop/start" mode or "constant-run" mode. If you're on the job and have a quick task to accomplish that doesn't require much air, then stop/start mode is sufficient. However, if you're using your compressor all day and have demanding requirements on it, you should operate the unit in constant-run mode, which will keep the motor running when it reaches max PSI. When this happens, air is "unloaded" through a vent port on the side of the Pilot Valve. until more air is called for, which then "loads" the pump to start pressurizing the air tank(s) again. Because the unit isn't constantly stopping and starting, this saves your motor and capacitors!