While I was stoking my woodstove I glanced under the stairs. My PW was handy so I snapped a pic of the unloader valve so you can recognise it, (if you didnt already know). Yours may well be different, pictured is what I refer to as a typical unloader valve.
The screwdriver is pointing at the retaining clip which when removed will allow you to pull the valve out. It'll probably be quite stiff in the pump housing and you'll very likely need to work it back & forth to slide it out. Be mindful of the valves o-rings.
Once out you may find it's simply sticking, cleaning it while working by hand with an oil or penetrating fluid 'might' free it up and provide it with a bit more life. If really jammed valve replacement will be necessary.
It may not be your issue, but something that needs checking in regards to diagnosis. Good luck 👍
When I came in for lunch yesterday I thought I'd continue with a couple more pics and show anyone who may be wondering what the typical PW unloader valve itself looks like once out of the pump housing. I havent had it out in probably 2 years, so I look at the inspection as preventative maintenance.
With this unit, slide back & remove the protective cover. You can see the retaining clip pulled back. If a pair of channel locks wont allow you to pull the valve out, slip a flat blade screwdriver under the spring and gently pry against the housing, you should see valve movement allowing removal.
As you can see in the pic below, its a relatively simple device. The small dia pin should slide in and out freely using your fingertip. Be sure any ports are clean and debris free.
If you find the pin to be seized in place, you can try putting some weight behind the unit on something solid (cement floor or workbench as examples) to un-stick it. Sometimes it works and you can save it, sometimes not. Once free add a little spray lubricant on the pins shaft while working it in and out.
When re-installing add a smidge of grease to any o-rings your valve has to aid with installation. Keep the grease out of all port holes.
You will likely have to tap it to fully seat the valve into the housing far enough to re-install the retaining clip. Do so lightly so as not to damage the valve.
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