Purge valve leaking...

Here's the scoop.
Western Star with 56k miles on it, 2009 so it sat for a while, but is under full warranty. S60 Detroit
Got up last week and notice air pressure going up and down when I started the rig. Popped the hood and the purge valve was leaking a little after it blew off. No biggie, I got a new one and put it in. Also replaced the air dryer filter. Fixed.
Today back in cold weather (North Dakota) and it is leaking again! Figured I got a bad valve, so froze my butt off instead of waiting at the dealer etc and changed it again. Same result.
One last thing to try, changed the governor. Same result. Thought maybe the governor was letting the compressor run on, causing the purge valve to leak..
When shut off, the gauges are at @90psi when I start it the next morning, so it isn't leaking all the way down etc.
It's a Wabco mounted on the frame rail, can probbaly get the model number from the build sheet if I go digging.
What else can it be? WS dealer can get me in tomorrow, just as I have to load a blade here in Grand Forks. It's building air no problem, so not overly concerned about it right now, but it is bugging me non the less.
Thoughts?

Martin

 

The purge valve does it's spitting, which is normal per the book.
How hard is the unloader valve to change?

Thanks!

Martin

 

Thanks fella's. Looks like I will give it a shot. No handy ws dealers close by my house, so that is why I am screwing with it myself.

Martin

 

Great tip, thanks!
Just so I have it clear....
Remove the intake hose from the compressor, off the intake duct from the filters.
Start the truck and let it build up until it blows off at 120PSI.
Put my finger over the end of the hose and feel to see if there is pressure trying to come back up the hose to where my finger is.
That about it?
I will try to do that tomorrow, cold and dark now!
Cheers

Martin