well pressure tank and air

I have a big 120 gallon well pressure tank, and having problem with air in the garden hose when i use my pressure washer to cleaning, i thought i already purged all air from the garden hose, and after running it for a while, and a little air still there.
i notice my well-rite tank has a little valve at the top of the tank, not sure that is for pressure charging, or water/air separator valve, do i need to install such valve? or not to worry a bout? i don't use pressure washer a lot, just once in a while for cleaning.

Assuming your well pressure tank is a bladder type, the Schrader valve on top is to adjust the air pressure in the tank. There should be NO water on that side of the tank, and no air from that (hopefully intact) bladder getting to the other side into the water.

What may be happening is your water level in the well is too low and after continuous use, you start to suck air. Not good for the pump. Your pressure washer has its own pump, and may be pulling more volume than your pump can maintain, essentially sucking the tank dry.