Air leak

Pull the dash and start there!

 

The sound you hear behind the dash does not mean that’s where the leak is
That’s an indicator that you have a leak somewhere

When do you hear the leak? When trailer or truck brakes released or all the time?

 

Air leak under the dash

 

Charge up your trucks air system with a shop air compressor and do not start the truck. Block the tires, push in the parking brakes and start listening for leaks.

 

I'd start by cutting off the air to the trailer. Then get a bottle of soapy water and start spraying every air connection on the truck, as well as the rubber on the air bags. If you see bubbles, you want to fix that leak. If you still hear air under the dash after fixing the leaks, you'll have to get under the dash and check the connections. You should be able to use your hand around the fittings if you don't want to spray soapy water under there, and it will change the sound when your fingers run over the leak. If it sounds like it's near the foot pedals, check the pressure sensors in the firewall and pass through area. If it sounds like it's up high, check the dash valve, as well as any elbows or tees on the air lines. If your seats are air operated, don't forget to check the lines to them.

Do the same procedure for the trailer after supplying air to it. You'll also want to spray the gladhand seals and lines, and the protection valve on the tractor side of the lines. When the trailer leaks have been fixed, you'll want to go inside and listen for leaks under the dash again.

Then I would look at the parking brakes. Make sure the wheels are chocked and the truck cannot move. Release the tractor brakes (yellow knob in), cut supply to the trailer (red knob out). Now go under the truck and spray the brake chambers and the lines coming off of them. Fix or replace any parts that are leaking. You can do the same for the trailer if you didn't check them earlier(yellow knob out, red knob in).

Finally, I would check the service brakes. Make sure the wheels are chocked, and press both knobs in. Let the air build up to full, then hold down the brake pedal. Watch the gauge for your air tanks and make sure it's not losing air while you keep the brakes held down for a minute or so. If it is, you'll want to grab a helper, so that one of you can listen and spray the area while the other person keeps the brakes held down. You could probably do it yourself with the right tools and/or creativity, just make sure the wheels are chocked.

 

Every trucker should get an Ultrasonic Leak Detector. They work great and save hours of time.
I used mine on a friends truck recently, couldn’t find anything. Then while circling the truck a third time he had the drivers door open and I homed right in onto his parking brake valve. You still couldn’t hear it once I knew where it was, but the detector found it in a flash.
TruTrack™ Ultrasonic Leak Detector | Robinair

 

2013 Cascadia. Im having the same issue. While driving, the air will randomly fall down to the alarm level (about 60 psi) for about 10-15 secs then build back up. If im sitting still with the brakes set, air will fall like normal to about 100psi but not build up unless I tap the brake pedal a few time to "trick" the system to start building back up. Had the compressor checks, working fine. Only other thing ive noticed is that parking valves aren't popping out together when i have a trailer on me. having to pull them both. can also hear a faint hiss behind the dash