Air brake issue, hearing a leak while pressing the break peddle.

Bad brake can , or busted line.

 

You now get to play elimination logic. When you apply brakes it pressurizes the service brake system. This is all hoses valves and canisters. Start big is it the tractor or trailer. My second move would be to inspect the brakes looking for signs of inoperative wheel (rust on the drum is the biggest clue) hole in air line or bad diaphragm are your most likely suspects.

 

With the knob pushed in right?

 

If parking brakes are released, the first place I look is the blue line to the trailer.

 

If you get out of your truck with the knobs in, don't forget chocks. Been enough threads recently with truckers getting ran over by their own equipment. Even if you have the brake pedal depressed with a board or something, you don't want to be under a tire when it slips off the pedal.