Common air leak on a Freightliner Cascadia, pass through manifold or air manifold 

Common air leak on the Freightliner Cascadia, the pass through manifold or air manifold located in the driver side firewall just behind the foot brake valve. You can try to remove the air line and cut tip off and then reinsert the air line and see if that helps. But the problem is usually in the fitting itself which is NOT serviceable.
TOOLS needed: torx bit set, ratchet, extension, 1 and 1/16 deep socket and maybe flat screwdriver.
Takes about 1-2 hours to remove and install depending on your experience.
Super easy to do yourself.