Air leaks from parking break knobs

Set trailer. See if it still does it
If not. It’s on the trailer side and if it does. Then the tractor

Find someone to hit the brakes while you walk around the truck
You should be able to find it

 

I wiggled mine for 30 year's sometime it worked.
Not that kind of wiggling either lol.

 

It could also be a leveling valve, sometimes the fitting is actually leaking back where the airbags are, have you had difficulty airing up the airbags or the ride being rough when driving??

 

Easiest thing to do is park on level ground, chock the wheels so the truck won't move. Release the brakes and find something to push down on the brake pedal. Then go hunting for your leak.

Just keep in mind, air systems can be a bit tricky to diagnose. The leaking component isn't always the culprit since most air valves always sit with the exhaust port open when un-actuated. A component upstream could be leaking and pushing air out thru the open exhaust of an unrelated valve.

 

Yeah it's supposed to send air equally to your bags so they fill up when loaded and deflated when empty.....thats why I said is the truck rougher when driving than normal?? Because a bag not fully inflated when loaded will mean all the weight isn't supported by the suspension properly...

 

I press down on the pedal with a pipe if no one is around to press the pedal for me. Then I get a squirt bottle full of water and conduct my investigation. I have to do it with almost every trailer at my company.

 

The knobs don’t leak, if you hear air rushing sound through the knobs it’s 99 to 1 and air leak somewhere else. Usually a brake chamber. Very often the brake chamber hoses are zip tied with other hoses some of which are wrapped in a plastic chafed protective barrier which will eventually rub its way through one of your air chamber hoses. If that’s the case, you can get sone rubberized tape that is made specifically for that purpose, but it’s just to get you back to the yard or to get it fixed. Or you can use a heavy duty pair of vise grips to cramp off the cylinder that’s leaking or the hose. Zip ties will work as well. If you do that, you’ll be down one brake, but it’s no big deal if you’re going to get it fixed.