A pressure normal come in 1600 2500 4000 psi ranges for you Vinyl 1600. Cleaning harder 2500 taking paint off steel and cleaning steel along with having longer reach from end nozzle 4,000 psi.

In link below it help first time users as well one you have the distance from nozzle to surface to be clean is very important so want cause damage but get job done Every day use one you learn something new.

Here is a link that might be useful: Scroll to page 10

If your siding is green, you might need to spray some very diluted clorox on first, with a pump sprayer. Have always heard never put clorox in a power washer.

It looks like something that would control the volume of the water. Why not make a note of its current position and open and close it with the wand hand valve fully open and spraying.