Air is everywhere. Hydraulic fluid isn't. Trains, buses and tractor-trailers use air-brake systems so they don’t have to rely on the hydraulic fluid in car braking systems, which can run out in the event of a leak.
All of these types of transportation are weighed down by heavy passenger or cargo loads, so safety is of the most importance. A speeding locomotive that relied on hydraulic brakes would turn into a deadly steel bullet if the brake system suddenly busted a leak.
In 1869, an engineer named George Westinghouse realized the importance of safety in the relatively new railroad industry and invented the first triple-valve air-brake system for railcar use. Westinghouse’s system worked the opposite way of a direct air-brake system. The triple-valve system performed three functions, thus its name.
Let’s take a look at those functions.
1. Charging: The system must be pressurized with air before the brakes will release. At rest, the brakes remain engaged. Once the system reaches its operating pressure, the brakes are freed and ready to use.
2. Applying: As the brakes are applied, air pressure decreases. As the amount of air decreases, the valve allows air back into the reservoir tanks, while the brakes move to the applied position.
3. Releasing: Once the brakes are applied and the air escapes after braking, the increased pressure releases the brakes.
This diagram shows the condition of the brake cylinder, triple valve and auxiliary reservoir in the brake application position.
When brakes are pressed, the air in the Brake pipe is released which causes a low air pressure in the triple valve. This causes suction in the valve making the slider in the valve to slide right. The movement of the valve causes the pressurized air inside the Auxillary Reservoir to rush to the Brake Cylinder pressing the piston against the spring which ultimately applies pressure on the brake block making the wheel slow down/halt.
When the driver releases the brakes, pressure in the brake pipe rises and enters the triple valve on each car, pushing the slide valve provided inside the triple valve to the left. The movement of the slide valve allows a "feed groove" above it to open between the brake pipe and the auxiliary reservoir, and another connection below it to open between the brake cylinder and an exhaust port. The feed groove allows brake pipe air pressure to enter the auxiliary reservoir and it will recharge it until its pressure is the same as that in the brake pipe. At the same time, the connection at the bottom of the slide valve will allow any air pressure in the brake cylinder to escape through the exhaust port to atmosphere. As the air escapes, the spring in the cylinder will push the piston back and cause the brake blocks to be removed from contact with the wheels. The train brakes are now released and the auxiliary reservoirs are being replenished ready for another brake application. In this way the train air brakes work.