I would start by sitting back a second and figuring out who you trust.
From my limited view, it would be the shop that rebuilt the valves, at least you trusted them enough to hand them money.
I would walk back in there with my hat in my hand, and ask for help, my conversation would go something along the lines of, Hey guys, I troubleshot this thing and thought it was the valves, so I had you guys rebuild it and it still is not working. I am over my head at this point sorting through this thing, what do you guys recommend I do? Then follow their advice.
I chose not to answer last night when I read about suing because nothing I would have said would be fit to print, but enough others have said it much nicer with more clarity than I could muster.
If you came into my shop threatening to sue, we would not get far together.