Air leak down when truck is cold

My experience has been when the leak slows/stops after the truck warms-including the HVAC air-its a leak at an airline under the dash at a quick connect fitting. Once the airlines soften up after warming, the fitting and plastic seal better. Air pressure dropping down that fast leads me to think it's a larger airline, like at the brake release valve. But I would think you'd hear that leak. If it is an at a quick connect fitting, cutting off the end at the bite mark (from the fitting) at the end of the airline usually gets rid of the leak-for a little while anyhow. I've gone thru this for years because I've only owned older trucks. I now just replace quick connects with compression fittings. Other than that, you just have to spray soapy water from a spray bottle all over the trucks airlines and where it bubbles is a leak.

 

It should be easy to hear with the truck shut off after the air pressure is built up, with that amount of air leak.