If you're at the point of buttoning the brakes up already, I'd just drive it without the prop valve to see what you think.
If you're waiting for it so you can finish plumbing though, I would still take it apart and look inside.
They might look fine outside, and no they don't fail that often. But they can fall prey to rust inside. Just like any other brake part.
Pull the warning lamp switch out, pull the delay valve out (nipple thingy with the rubber boot over it) and pull the plug out of one end so you can see if the shuttle piston can come out or is stuck inside.
In other words, do a thorough inspection rather than just soak it. Let some light inside to see what crawls out.
Paul