Answer :
Final answer:
The test for the tractor protection system requires the trailer supply valve to close before the pressure falls below 55 psi to ensure safety in the event of an air supply issue.
Explanation:
The question pertains to the testing of the tractor protection system within a commercial vehicle's air brake system, specifically related to the trailer supply valve. When testing this system, it is crucial that the trailer supply valve closes off the air supply to prevent tractor-trailer separation if an air line is damaged or disconnected. According to regulations, this valve must close before the pressure drops below a certain point to ensure safety.
In the context of the options provided, the correct answer is that the trailer supply valve must close before the pressure drops below: 55 psi.
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