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Question:

According to federal regulations on air brakes, the brake linings of drum brakes must be at least _______ thick.

A 1/4 inch
Explaination

According to federal regulations on air brakes, the brake linings of non-steering-axle brakes must be at least 1/8 inch thick for disc brakes or at least 1/4 inch thick for drum brakes. The brake linings of steering-axle drum brakes can be thinner.