What would cause the pressure to be low in my DeVilbiss/Excell XR2500?

Does it use unleaded gas?

As soon as I turn the water on after hooking a hose to the unit, water begins to spray out what looks like to be some sort of relief valve. It is spring loaded and is white plastic. If I push in on it, the water flow almost stops but as soon as I let it back out, it begins to spray. I didn't see any such part listed in the parts list.

There is water spraying out of the chemical inlet valve.

When the pressure washer is on with high pressure tip in wand, a stream of water comes out of the siphon hose. Is it in the pump or the spring and ball in the siphon inlet? Thanks

How often do I change the oil and what oil should I use? Also, how much oil?

I am getting some pressure but not enough to do the job. Is my pump gone or maybe something minor needs to be replaced?

I replaced the o-rings in the cold water pressure washer and I still don't have pressure. Could it be the valves?

One of the faults of this washer is the pump, which has two straps that are on the top and bottom and move the pistons. Fatigue makes these weak over a couple of years of occasional use; and when one breaks, it can easily break one of the pistons. They appear to be ceramic, and break easily to shearing pressures caused by one strap breaking. From my 'net research, this is fairly common. I ordered straps (both) and a piston back in early September, and it is now December 26th. I received the two straps fast, but the piston has been backordered since I placed the order. I would recommend a different pump if you can't afford to wait for parts.