I recently had to replace my D2 valve when the original quit working. Being curious, I decided to disassemble the old one just to see what she all about. Turns out, there was no actual wear to the parts inside, just 25 years of built up grunge. After thoroughly cleaning all the bits and pieces, I reassembled it. I believe that I now have a refurbished, functional air governor spare.
Richard
I did the same thing with mine but also replaced the o-rings and lubed with silicone grease--carry it for a back-up
Chris
Do the same clean, new o ring, lub, for the step slide and drop-down step pneumatic 12v valve when those valves slow down, leak or fail. No reason to buy replacement for step valves.