You might want to check for a clogged debris screen , if you are using them.
I think that valve is available for under 20 us dollars new, online.
Not worth much trouble to repair, I suspect a restriction in the line rather that a bad valve.
If you buy a new one, just too try it, and it does not help , Then you would have a good spare.
Good Luck on your project.
Have a good Day.