The tractor protection valve closes automatically when air pressure drops to the range of 20 to 45 PSI. This prevents further loss of air from the tractor and applies the trailer emergency brakes.
The CDL (Commercial Driver License) exam tests your knowledge of commercial driving rules, safety procedures, and vehicle operation. You must score 80% or higher to pass the written knowledge test. The exam covers general knowledge, air brakes, combination vehicles, and various endorsements.
This question is from the CDL Combination Test #1 section. Practice with more questions from this category to build confidence before taking the official DMV test.
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