What holds the parking brakes on trucks when air pressure is removed?

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Prepare for the Alabama CDL Class A Permit Test. Study with flashcards and multiple-choice questions, each question includes hints and explanations. Ace your exam!

Multiple Choice

What holds the parking brakes on trucks when air pressure is removed?

Explanation:
Spring pressure holds the parking brakes on in heavy trucks with air brakes. These systems are designed so the brakes are spring-applied and air-released: a compression spring inside each brake chamber pushes the piston to apply the brakes when air pressure is removed. When you apply air pressure, it pushes the piston to compress the spring and release the brakes. If air pressure is lost, the spring expands and re-applies the brakes. Hydraulic pressure, vacuum, or electrical current aren’t what keep the parking brakes engaged in this standard setup.

Spring pressure holds the parking brakes on in heavy trucks with air brakes. These systems are designed so the brakes are spring-applied and air-released: a compression spring inside each brake chamber pushes the piston to apply the brakes when air pressure is removed. When you apply air pressure, it pushes the piston to compress the spring and release the brakes. If air pressure is lost, the spring expands and re-applies the brakes. Hydraulic pressure, vacuum, or electrical current aren’t what keep the parking brakes engaged in this standard setup.

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