See the red button on the filter. Used to remove air when filling. The filter is on the suction side of the pump. When the pump starts, the path of least resistance is thru the red button. It gets pulled down slightly, air enters, and when the air hits the impeller it becomes millions of little bubbles. The pump loses efficiency with all those bubbles and may not be able to reach shut off pressure. If you have an accumulator tank, it's probably half full of air on the water side and not operating correctly. It can cause the pump to have short cycle.”
I like it! Air getting into the intake side. Explains everything. Move the filter to the pressure side, maybe install a proper strainer on the intake side and everything should work. Nice catch on the vent button.