snype,
great, glad you finally found it.
yes sometimes these things can be difficult to find even for those that are trained and work on them everyday.
The fact you found a piece of steel lodged in the check valve sort of concerns me that it had to come from somewhere but...
unless you find alot of it then i wouldn't worry to much about that, it could of been a piece of machined metal left inside a manifold block when it was made and was just floating around till it got caught up in that valve. Lets hope so anyway :o)

yes as far as that other problem checking the other valves in that circuit is a good idea, you may find another stuck by debris. If it does it consistently then that shouldn't be too hard to find.

good luck