Losing pressure on my machine

I have a 3500 psi and 5 gpm machine. When running and not on the gun I have the correct pressure. As soon as I get on the gun I lose just about all pressure. I’ve tried it with one 50ft hose and same thing. Any ideas of what may be wrong?

Is the unloader cycling? Is that the original shipping plug on the pump?

How can I tell if the unloaded is cycling?

Not original plug. That’s the one they shipped with the machine that I had to put on it after shipping.

I think this is the problem. Can i use WD-40 and lube it up or do i replace it?

Replace it. Buy a couple so you will have a spare.

Will do. Is this common?

Thanks again

Nothing lasts forever. A little piece of trash can make the shutter valve stick.