Blowoff occurs at about 125 psi. The engine driven air compressor will kick back in about 105-110. For other readers, as I know that Tom is aware of the this, it is important to watch you air pressure gauges. If the supply air gauge is dropping more than 20 psi in less than 40 minutes while the engine is off and no air using equipment (generator slide, air bay slides, air operated doors, leveling, etc) is used you should begin checking for air leaks. If either or both of the brake air gauge needles are dropping with the supply air gauge, you have a dangerous situation and need to immediately replace the check valves going into the two brake air tanks. The brakes air pressure should hold air for an extended time (certainly most of a day at least) before dropping 20 psi unless you step on the brake or activate/release the emergency brake.
A leak in the supply air system is a frustration, a leak in the brake air system can ruin the rest of your life.
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