Lift kit problem

Anyone know why you'd get air returning to tractor through the lift kit line to the trailer. I can hear it through the dash air rocker switch? Truck is 2018 389 and trailer is Cottrell cx09. The hissing sounds like an air leak at the switch but when I took the dash panel off no leaks . When I disconnect the lift kit air line the hissing stops. Also, when i turn the lift switch on the air hissing sound stops. No leaks on the trailer suspension after soaping it up. I'm puzzled. Any thoughts would be greatly appreciated.

 

Ok thank you. I'll try that.

 

I replaced the switch and still have the same problem. Puzzling the heck out of me.

 

Turns out is was the pilot valve on the trailer leaking internally feeding back air through the dash switch.