Answer :
Final answer:
The air supply pressure gauge should read 120 PSI before backing under a trailer.
Explanation:
The air supply pressure gauge should read 120 PSI before backing under a trailer.
When backing under a trailer, the air brakes of the truck need to be engaged. The air pressure in the system needs to be sufficient to activate the brakes. The recommended minimum air pressure for backing under a trailer is 120 PSI. This ensures that the brakes will operate effectively.
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