When checking the air loss rate in a Class 1 vehicle with the engine off, the parking brakes released, and the service brakes fully applied, what is the maximum acceptable leakage rate?

1. No more than 4 psi (28 kPa) per minute for a tractor-trailer combination
2. No more than 10 psi (70 kPa) per minute
3. Zero leakage is allowed under any circumstances
4. No more than 1 psi per minute