How do you find air leak on tractor trailer?

First, is it audible? If so, you simply follow the sound. Once you have it isolated to a general area, you spray off all air brake component systems with a mixture of soapy water... where you see bubbles forming is where you air leak is. The circumstances in which your air leaks out is important, as well... if you have a Cummins engine, air is supplied to the fan hub with the key in the on position... if that's the circumstances in which you're losing air, spray the fan clutch and hub... if you see bubbles forming at the fan clutch, it needs to be replaced. Push-in air fittings are vulnerable.. check those. Any point where air lines appear to be rubbing against something should be checked, the leveling valve and air line fittings on top of the transmission are traditional weak points, if you're losing air with the brakes released, check your brake chambers... if you hear air coming out of them, the boot is compromised, and you'll need to either replace the boot or the entire brake chamber. If you're hearing a constant surge of air when you have your trailer supply valve pushed in, check both that valve and also the double check valve under the cab. Inspect the air bags - both cab and suspension. Basically, spray down anything which has anything to do with air. With the 72 hour annual DOT inspection currently in effect at the time of this posting (0106 MTZ, 2 June 2015), you REALLY don't want to risk running with anything which could get you put OOS.