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When preparing for a road test, upgrading drivers must know when the air brake system is at maximum pressure. By know that the air brake system is at maximum pressure, the student is able to determine if the governor is working at its maximum pressure.

There are two symptoms that tell a driver that the air brake system is at full pressure:

1) I HEAR: The air dryer purges (on some newer trucks these aren't loud, so you'll have to roll down the window to hear)

2) I SEE: The air gauge needles have stopped climbing between 100psi (690kPa) and 135psi (931kPa).

If these symptoms are present, than the driver knows that the governor has put the compressor into the unload or cut-out phase.