Seeking Advice on Air Leak/back-feed Issue with 2017 Freightliner Cascadia

Hey everyone,

I’m running out of options and could really use your input on an issue I’m facing with my 2017 Freightliner Cascadia. The truck came from the Schneider fleet, possibly from the tanker division if that helps.

Here’s the problem: whenever I hit the brakes, I get an air leak from behind the dashboard, specifically from the area connected to the trailer disconnect or trailer hold switch. This only happens when a trailer is connected.

We’ve checked everywhere and couldn’t find any air leaks except for this one. We’ve replaced every valve and check valve but still haven’t located the source or cause of the leak. The ABS modules seem fine
– no issues there.

Here comes the detailed part:

• Line number one (maroon line on the top) gets air backfeed when I push the brakes.and goes to dashboard switch i added the picture of that switch
• Air goes through line number four (black line) back to the trailer when i push brake and comes back through the red line on the left side of the picture.
• Line number three is receiving the backflow. If I disconnect this line between the check valves , I cannot disengage the trailer parking brakes, which locks the trailer downeven dough red knob in
• I haven’t checked if line number two backfeeds. The check valve on that line number two has also been replacedalong with every other.
• According to Freightliner, when I push the switch, air should come out of both the blue and red lines going to the trailer, but when I engage the switch, air only comes out of the red line.

Has anyone ever come across an issue like this? Any advice or suggestions would be greatly appreciated!

Thanks in advance!