Western Star with Mercedes

I've got an 07 WS with an MBE4000 (EPA04) engine. My dryer purge was acting up so I installed a purge valve. Within 20 miles...it was still sticking open once in awhile.
So I changed the Ad-9 dryer for a new one and its been about 3 weeks and the purge is acting up again.
There doesn't seem to be any oil coming out of the dryer. When I changed the purge valve it was a little dirty but not really oily. More like a black soot.
I've sprayed the lines and there doesn't seem to be any leaks.
Compressor is a bit of a pain to get to because it's on the back corner of the motor but it still builds air fairly well. Except when the purge valve sticks....then it usually will sit at 80-90

 

The compressor seems to be working ok so I never really considered the governor. But it's a cheap enough part...maybe I'll just throw one on and see if it helps.