I had referanced a pre trip inspection in my first reply. Below is the critical part of teh Air Brake check. As long as you pass this test and you do not hear and air leak ... I would say that you are good to go
Ensure all drain cocks are closed and bring system pressure to its maximum.
• Check to see that the low air pressure warning devices are working as required.
They should cut out at a minimum of 60 PSI.
• Check to see how long it takes to build up pressure from 50 to 90 PSI.
It must take less than 3 minutes with the engine running at 1200 RPM.
• Release tractor and trailer brakes and continue building system to maximum pressure.
• Check to see at what point the governor cuts out.
This pressure should be in the range of 120 to 130 PSI.
Someone asked if it is normal to hear a rush of air when putting down the jack. Yes ... at this point you are dumping all air from the system. This lowers the coach making the leveling process much easier ... also preventing that huge first step when level