Do Proportioning Valves Wear Out?

Reproductions are available for $100 at Classic Industries. Not sure of quality though.

http://www.classicindustries.com/mopar/parts/md2268.html

I bought a used one off this site not realizing aftermarket valves were available, and not wanting to have to mess with plumbing an adjustable one. Turns out the rear port on the used one was plugged.