Final answer:
The force keeping the air brakes released is supplied by the compressed air maintained by an onboard air compressor. Air brakes, first introduced by George Westinghouse in 1869, rely on compressed air to control braking in heavy vehicles like trains and trucks.
Step-by-step explanation:
When the air brakes are released, the force which keeps them released is supplied by the compressed air in the system. Air brake systems, such as those found in locomotives and heavy trucks, utilize compressed air to apply and release the brakes. The invention of the air brake system by George Westinghouse in 1869 allowed for much safer and more efficient operation of trains, as it enabled the engineer to control the brakes from the locomotive rather than having workers manually apply brakes, which was a dangerous task.
In a modern air brake system, when the driver wants to release the brakes, they would typically decrease the brake system's air pressure, which allows springs within the brake mechanism to retract and release the brakes. To keep the brakes released, the system must maintain sufficient air pressure; this pressure is usually supplied by an onboard air compressor. Therefore, when the air brakes are off, it is the air compressor that continuously supplies the compressed air to counteract the mechanical springs and keep the brakes from engaging.