It is normal for air to exhaust as you release your foot from the pedal. If you are holding the brake pedal down steady and you hear air leaking THAT is bad.
For those who missed this procedure before it is definitely worth repeating.
Testing air pressure and looking for leaks as a standard pre-trip check. If you want to be sure you are safe you should be doing this every time you start a day.
First:
Build up air till the air dryer goes off and the compressor stops building pressure, with the parking brakes on. Wait 10 minutes and record the drop.
If you have a loss here it is going to be aux. air or possibly service air too.
Next:
Now first release the parking brakes pump the brake pedal to get the pressure down far enough to get the compressor pumping air and again build up to 120. Wait 10 minutes and again record the drop but this time you have the parking brakes off.. (The DOT allows up to but not more than 20psi drop in 10 min.)
If an additional drop is observed from the initial test you have an emergency air leak. ( probably a maxi or a line going to it)
Finally:
again with parking brake released and air built up to 120 step on the brake pedal and hold. Turn the bus off and open a window to help you listen along with watching the gauges.
If you hear air leaking but only when you are applying brake pressure it is Service air. (IMO anything heard during this test is unacceptable)
In conclusion the easiest way to completely rid yourself of pressure loss is to first get it to hold air with the parking brake on. When you get to there then release the parking brake and again get it to hold.
I can't tell you how many times I have got to the end of a search confident I have finally found all the leaks only to come out the next day and see pressure loss, go over things again and find more. This project is a lesson in perseverance and one worth doing for sure.
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