93 w900 not purging air

Probably the original stratoflex lines to the governor, and leaking.

When does it quit building?

 

You need to plumb shop air directly to the compressor outlet, and check for leaks with the truck off. If it doesn't leak, bypass the dryer.

You've got a simple issue here. Don't expect to be able to hear a leak, trust me.

Make sure your governor is plumbed correctly too. Supply side should be to an air tank, signal side to the dryer. When it hits pop off pressure, you should get a signal to your dryer from the governor.

 

Bypass the air dryer and see if it builds faster.

Did you notice air pressure come up quickly with shop air?

 

I doubt this has anything to do with your problem, especially with a new compressor, but I have had this valve (unloader?) get gummed up and stick so that it doesn't start to build again.
@Big Road Skateboard knows much more than I about this, but it only takes a second to pop it off and make sure it's not gummed up.

 

What kind of dryer is on it and does it have a. E con valve off the governor or is it just signal/purge line?