One of the things that irritated me the most when I was a mechanic was getting a truck to work on and having to build air pressure due to leaks. Once I got caught up with other repairs, air leaks became my focus.
Some can be heard, in a quiet place, brakes set, wheels chocked, you can crawl under the truck to find a leak, then repair it. Those leaks that cannot be heard can be found using a soap solution, spray on fittings and other air connections watching for bubbles.
Once the leak is found, the repair can be made. Remember, safety first, these airlines are under pressure. Drain all air before disconnecting any lines or removing any fittings.
Till next time, be safe out there.