2011 367 that wouldn’t build air this morning

Hose between air compressor and air tank builds up carbon and plugged up ?
The stainless steel one...

 

The problem is intermittent so highly unlikely to be anything plugged or an air leak. Intermittent building of air is far more likely to be an unloader valve problem.

 

front1.fm (wabco-customercentre.com)
The link is for Wabco on a Mack (I know you don't have a Mack but compressor likely to be similar). Does yours look anything like the pic on the cover page? Pg 2 and 9 talk about the unloader and leaf valve which are internal to the compressor.
Reality is that you have a 13 year old compressor that is going to need replacement. Yes check the output line.
Call Wabco and ask for technical help, tell them your problem and ask about the unloader on your model.

How to Obtain Additional Maintenance and Service Information Visit our Literature Center at wabco-na.com/literature to access and order additional information. Contact WABCO Customer Care Center at 855-288-3203 (United States and Canada); 800-953-0248 (Mexico); or email wnacustomercare@wabco-auto.com WABCO Maintenance Manual 31 (Revised 03.20

 

I just went through this a week ago.
Unhook the line from compressor to dryer, 9/16 on mine.
have truck off, air will pop out and drain from dryer, pump brakes down so no sit in tanks.

Start truck(line still not hooked up) see if it builds air.
Hint, may not go over 100 both tanks, it’ll be blowing out line to dryer.

If building, reattach line.
I tried changing i loader valve, and cartridge, it didn’t help, i put a new dryer on, seems to be working, i haven’t had to stop on ramp in the cold to do the steps.

 

Good that what you did worked for you. Here's why his case is different. Still would not hurt to try what you did. He drained the air and it would not build. Disconnecting the small line from the dryer to the compressor will not do anything as there is no air (at least should not be) to keep the unloader open. When things work properly and air builds to the governor cut off pressure (lets says 120psi) air returns through the line at the dryer back to the compressor and opens the unloader valves so air is not pumped out the discharge line to the dryer. Unless air is somehow trapped in the line holding the unloader valves open even though the tanks are empty. Draining the tanks and pumping the brakes then disconnecting the small line at the compressor to see if there is air would be telling.

 

i drained air too, but until i unhooked line(still had pressure) you heard it close(truck off)
i had to unhook that line once a day for a week til i got home, maybe it’ll work, saved a service truck

 

So in your case air was holding the unloader open when it should not have due to a problem in the dryer. Air was trapped in the line and not being released. As you said changing the unloader did not solve the problem because the unloader was not sticking but the air was keeping it open. What's odd is that you only had to disconnect the line once per day.