How can you check that the spring brakes come on automatically?

To check that the spring brakes come on automatically, the most effective method is to pump the brake pedal to reduce air pressure. As the air pressure in the brake system decreases, there will be a point at which the pressure drops below the threshold required to keep the spring brakes released. When this happens, the spring brakes will engage automatically. This action is crucial for safety, as it means that if there is a loss of air pressure, the spring brakes will activate, preventing uncontrolled movement of the vehicle.

The other options do not effectively demonstrate whether the spring brakes will activate. Increasing the load on the vehicle does not directly relate to the functioning of the spring brakes, as it primarily affects the vehicle's weight and handling rather than the brake system's air pressure. Checking the brake fluid level is more relevant for hydraulic brake systems and does not indicate the condition or function of spring brakes, which rely on air pressure. Engaging the emergency brake handle may engage the brakes but does not test the automatic engagement feature under conditions of reduced air pressure, which is the critical aspect of the question.