W900 air leak only when driving

Hi I have a 2007 w900 and it leaks air at 110psi and won't stop until I let off the throttle. Then it builds pressure normal and spits at 130psi like it should. Everytime it goes down to 110psi it leaks again. Usually if I'm light on the throttle it's just a quick hiss and goes away. Anyone know what it could be. I can't find no leaks when truck is parked.

 

I pull company trailers so I switch often. It's done it with everyone so far. The governor was my first thought too. I'm probably gonna replace it when I get back home. The hissing sounds like it's coming from the drier area but can't duplicate the problem when stopped.

 

Unloader and purge valve on the drier, right?

 

Thanks I guess I have some work to do this weekend.

 

So you're saying boost is leaking into my air system and forces purge valve to open? It sounds like you done that before. Is rebuilding the compressor top end an easy job? I do have plenty of tools from wrenching on cars. Wrenching on trucks is new to me tho. I just bought this truck last month and want to work all the kinks out.