The air leakage rate for a combination vehicle should be no more than how many psi per minute when the engine is off and the brakes are off?

The air leakage rate for a combination vehicle should be no more than 3 psi per minute when the engine is off and the brakes are off. This standard is in place to ensure that the vehicle's air brake system maintains sufficient pressure to operate effectively.

If the leakage rate exceeds this limit, it indicates potential issues within the air system, such as leaks in hoses, fittings, or the brake chambers, which could compromise braking performance. Ensuring an acceptable air leakage rate is crucial for safety, as it guarantees that the air brake system can build and maintain adequate pressure for safe and reliable operation.

The other options represent higher leakage rates that would not be acceptable according to safety standards for air brake systems in combination vehicles.