Air valve keeps going bad

If that's a bendix valve and its that colour it's a reman

 

My guess is the issue is with the relay valves themselves. If it was air system related, you should have seen that issue with other valves. Likely poor quality reman. Most of the air valves on my A worked just fine even after sitting for a decade.

 

I'm thinking the valves leaking may be a symptom of a faulty compressor or dryer? I've seen desiccant from the air dryer cartridge cause this as well as excessive oil from the compressor causing the rubber internals swell and cause a leak.

 

Well if the issue goes away once you replace that valve I would start looking upstream of that valve. Either something is getting pushed through the air system (like oil or water) and it is affecting that valve. Does your truck have an alcohol evaporator/injector? What about air system pressure? Too high?

Also is that the service brake or spring brake relay?