karcher 2.94m leaks water when lance released?

Hi I have a 2.94M Karcher washer I have bought parts last week , most of the moving parts had disintegrated inc springs due to the path cleaner which is acid, being sent through it . Control valve etc replaced and the green valve in the inlet pipe, put it back together and it works but when the lance is released water squirts out of the very small square hole in between the outlet pipe going to the lance and the control body( the casing around the control valve which switches machine on and off as lance is pressed. Looking down the inside of the outlet pipe is a pinkish nylon something or other with a rubber seal on its outer edge , I cant get this out although it has a slot for a flat blade screwdriver I t just turns, I am guessing this part is faulty as it’s the only part near where the water squirts out. Any idea what this part is how I get it out and what it is called so I can try and get one cheers Robin