Which factors determine the crack pressures on an air brake system relay valve?

The crack pressures in an air brake system relay valve are influenced primarily by how heavily loaded the axle is and the size of the axle's brakes. When an axle is heavily loaded, it requires a greater amount of air pressure to engage the brakes effectively, which in turn affects the crack pressure settings of the relay valve. This is because the relay valve is designed to ensure that air pressure reaches the brakes in a manner proportional to the needs of the system.

In addition, the size of the axle's brakes impacts the required pressure to initiate brake application. Larger brakes can dissipate heat more effectively and require less pressure to achieve the desired braking force, while smaller brakes might demand a higher crack pressure to engage at the same level of performance. Therefore, the interplay between load and brake size is crucial in determining the crack pressures of the relay valve.

The other options, while relevant to overall system performance and function, do not directly influence crack pressure settings in the same significant manner. For instance, air supply temperature and conditions of the brake pedal can affect system efficiency and function, but they are not primary factors in determining the specific crack pressures set within the relay valve. The length of the brake lines can contribute to pressure loss but doesn't fundamentally impact the relay valve's