Air Leaking from yellow line
 

Air leaking from yellow line on firewall (when brakes applied). Tractor Protection Valve replaced, no difference.

Find the leak and fix it.

I'm not trying to be funny, but if a line is leaking, it's either got a hole in it, or the fitting where it connects is loose or defective.

If your truck has those "push in" type connectors, sometimes if there is tension on the line they can work loose after a while. Try pushing the line back in to the connector and see if the leak stops. If so, you've found the problem.

When I encounter that situation, I add a few inches to the line, using a compression coupling and a short piece of line. That takes the tension off the push in connector so it does not try to pull out again.

The yellow line connected to what? Lots of yellow line...

Just one? one leak? and you know where the leak is? are you kidding? I have had leaks for years & still can't find em all. if u know it's from a certain section of yellow air line, replace or repair it. not trying to be a smart azz. if line was leaking from firewall, why did you replace t.p,v ? let us have some more specific info, so we may help.

I'm assuming the yellow line is open (not connected to anything)? does this happen while the trailer spring brakes are applied or released, or both?

This is happening with the brakes released and when you make an application?
If the yellow line is open then it's hooked to the exhaust port in your dash valve. Could be a faulty dash valve or foot valve since it's happening on application.

First though I would check that it's not a bad brake pot feeding back. You can check that by pinching off the emergency side (spring brake) line at each pot one at a time and making a brake application. I've seen feedback from a bad pot cause some odd problems.

Could also be a bad anti-compound valve. Most (if not all) are built into the relay valve. If you can determine which line it is at the valve you could plug it off to test.

If the pushrod seal in the brake pot is leaking,the park air usually leaks out the exhaust port of the service relay,which should stop when you apply service brake.the anti-compound line is usually 1/4"synflex tubing from the service relay DEL port to the park relay BAL port... again,a leak past the BAL diaphragm should stop when service brake is applied.

air leak
 

99% of the time you are looking at a bad break champber causing the feedback thru the yellow exh line. It can be the vlave..but those usually ffedback in the ery cold weather.

just air it up , shut down the engine and listen very carefully to each bakechamber