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How It Works: Piloted Unloader Valve

The Piloted Unloader Valve

Piloted unloader valve 131B

There are few components on an air compressor that can multi-task quite like the piloted unloader valve. This single component can activate an engine idle control, divert air from the tank when it reaches the top pressure setting and can reduce oil consumption by equalizing pressure between the pump and check valve. The following blog post will help you understand the piloted unloader valve by pointing out its components, explaining how it works and describing the adjustment process.


The diagram below shows the components of our most common piloted unloader valve, part number 131B.

Piloted unloader valve diagram

How It Works

Piloted unloader valves combine a pilot valve and an unloader valve to run an air compressor continuously. While this type of valve is most commonly used on gas-powered air compressors, it can also be used on electric models. While the air compressor is running, the pilot remains closed until the pressure reaches the unload setting. When that happens, the pilot opens and pressurizes the unloader valve, causing the unloader valve to open and excess air from the compressor to vent to atmosphere. As the compressed air is used and the pressure drops to the load setting, the pilot closes and depressurizes the unloader valve. The unloader valve then closes and the pressure begins to build again and repeats the cycle.

How To Adjust

For a visual guide on adjusting our piloted unloader valve, click here. Otherwise, read on.

Resetting Differential

  1. Make sure the top toggle lever is in the down (horizontal) position.
  2. Loosen locknut (1) by turning counterclockwise several full turns.
  3. Gently turn differential setting (2) clockwise only until the internal rod makes contact with the steel ball inside.
  4. After making contact, turn differential (2) counterclockwise exactly 1/4 turn.
  5. Secure position by tightening locknut (1) and proceed to top end setting.

Top End SettingPiloted Unloader Valve Adjustments

  1. For the standard top-end setting of 135 PSI, adjust differential (4) so that 3 threads show above locknut (3).
  2. Loosen locknut (3) by turning counterclockwise several turns.
  3. Turning (4) clockwise to increase the top end pressure setting or counterclockwise to decrease.
  4. Begin at a low setting by turning counterclockwise one full turn. Proceed to fine tuning instructions.

Fine Tuning Instructions

  1. Start engine/motor and observe tank pressure gauge. As tank pressure approaches 135 PSI, the unloader valve should begin unloading at felt muffler and cause gas engine to slow down to idle speed.
  2. If pilot valve does not unload as tank pressure approaches 135 PSI, slowly turn top end setting (4) counterclockwise until pilot starts to unload prematurely (at 105 PSI).
  3. Turn top end setting (4) clockwise in 1/4 turn increments, tap down on internal rod at toggle and repeat until desired top end setting is reached.
  4. If pilot valve begins to rifle (sputter), fine tune differential setting (2) 1/16 turn in either direction until rifling stops.
  5. After fine tuning, secure setting by tightening locknuts (1 & 3) while holding settings (2 & 4).



8 thoughts on “How It Works: Piloted Unloader Valve

  1. The unload valve on my emglo compressor is a different configuration but the adjusting Differential and top end adjustment look the same. I’ll try this and we if it works.
    Thanks, Dave

  2. I have an Emglo 60 gallon upright electric compressor. It has a pilot valve on it. Would a compressor with a pilot valve also have a check valve?? I’ve looked all over the compressor and cannot find a check valve

    • Hi! Thanks for reading our post. It sounds like you might have a constant speed head unloaded model. The compressor will constantly cycle while turned on and dump head pressure from head unloaders located at the top of the pump. There is likely no check valve in that configuration.

  3. When my control valve reaches the cutout pressure. All the air in the tank is released and the engine stalls? Is my valve faulty or could there be some kind of obstruction?

    • Hi David,

      Sorry for the delayed response! It sounds like there is some debris stuck in the check valve preventing it from sealing. If you follow the discharge tube from the head of the pump down to where it enters the tank, that’s where your check valve is located. You should be able to remove it and clean out the debris. Just be sure to shut the compressor down and drain the tanks before doing so.

      If you have any additional questions or concerns, email me at coreyn@rolair.com.

  4. On my rolair, soon as the tanks are full and the engine goes to idle, a ton of air exits from the discharge exhaust. Anyway to fix or adjust this? Thank you

    • Hi Zach,

      It sounds like the check valve is leaking, likely due to some debris preventing it from sealing properly. If you can provide a model number, I can point out on a schematic where the check valve is located. On its own it’s a minor issue that’s easy to fix, but it can cause damage to the head gaskets if left unresolved.

    • Hi Zachary,

      Sorry for the delayed reply. Somehow your comment got past me.

      Air is supposed to discharge from the piloted unloader valve when the compressor goes to idle. Since the engine is still running, the pump is still producing air. That excess air gets discharged to atmosphere until the tank gets depleted and needs to be recharged.

      Are you losing tank pressure rapidly without using the gun/tool? If that’s the case, I’d remove the check valve and check to see if something is preventing it from sealing. Of course, be sure to shut down the compressor and drain all tank pressure before doing so.

      If you need help locating the check valve, email me at coreyn@rolair.com with the model number.

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