Piston Airless Valve Diagnosis

Diagnosing problems with airless piston pump pressure valves.

Here is a way to test your piston airless spray pump to ensure the inlet valve and outlet valves, including the relief valve are functioning correctly.

This test is much easier to do and see the results with a large longer stroking airless piston pump compared to a small short stroke fast cycling piston airless sprayer.

A small airless spray tip that slows the output of the airless piston pump will make this test easier to read.

Prime the airless piston pump, usually with water is the easiest, and once the pump is primed close any bypass valve and wait for the pump to stall.

If the pump does not stall begin to spray until you are sure there is no air left in the airless paint spray hose.

Stop spraying and now if the airless pump still does not stall check the return hose or pipe.

If there is still fluid running from the return then the bypass or prime spray valve is the first problem.

The valve could be just dirty or blocked open with dried paint or other foreign matter or it could be worn, chipped, bent or broken.

A leaking bypass valve on any piston airless spray pump will need to be replaced or rectified before proceeding with further testing.

If the bypass valve on the airless sprayer is shutting off completely and the pump is still cycling then there must be a problem with the inlet and or outlet valve in the paint pump.

Carefully watch the travel of the piston.

With a piston airless paint spray machine the top ball and seat which is located in the piston needs to be closed and sealed on the up stroke.

On the down stroke in a piston airless sprayer the bottom ball and seat needs to be closed and sealed.

Trigger the airless spray gun which is fitted with the smallest airless spray tip available in your kit on and off as quickly as possible.

Watch the movement of the piston as the airless gun is triggered on and off. The piston should always move and stop anywhere along either the up stroke or the down stroke.

For example let's say the top ball and seat or outlet ball and seat is leaking. The piston will stall in any position along the down stroke. As soon as the piston passes the changeover point at the bottom of the down stroke the piston will move immediately all the way up to the extremity of the up stroke and then stall somewhere in the down stroke.

This indicates that the upper or outlet ball and seat in the airless piston pump is not sealing and that the bottom ball and seat or inlet valve is okay for this test.

If it is the inlet ball and seat or lower seat and ball that is not sealing you may notice that the pump will never stall with the piston travelling in the down stroke direction and will only stall when the piston is travelling upwards when the outlet ball and seat is sealing.

The causes of an upper ball and seat or lower ball and seat, inlet and outlet valve, in an airless piston pump not sealing could be any one combination of wear, corrosion/erosion, chips or foreign matter etc. preventing the ball from sealing in the seat.

Further inspection will be required through removal of the faulty valve for visual inspection.

  • Bypass
  • Piston Airless
  • Spray Gun
  • Spray
  • Valves
  • Troubleshooting

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