The Importance of Quick Release Valve

Quick release valves are an important component of many air brake systems. They reduce the time required from the application of the brake pedal to the time it takes for the air pressure to disperse out of the brake chambers into the atmosphere. They are a necessary part of reducing brake lag and preventing compounded braking.

The quick release valve is located near the brake chamber and is designed to discharge the air that reaches it faster than if it went through the relay valve's exhaust port. As the supply pressure drops at the quick release valve, the air in the brake chamber is released out of the bottom of the valve rather than allowing it to travel all the way back to the relay valve's exhaust port.

Some air brakes have quick release valves mounted closer to the brake chamber than the relay valve. This helps to release the air from the brake chambers quickly and prevents compounded braking as well as reducing brake lining wear and drum overheating.

A quick release valve can be found on both the front and rear of a vehicle. They are typically used on long wheelbase tractors and trucks.

They also can be found on some transit and inter-city buses. They are usually connected in the line between a system park valve and a paring brake relay valve.

These valves normally open to allow air pressure to flow from the supply port to the delivery port and are closed when the brake pedal is depressed. This allows the air pressure from the rear of the vehicle to actuate the front brakes while limiting air pressure from the front of the vehicle to actuate the rear brakes.

This valve is also known as a 'limiter' because it reduces air pressure to the front brakes (providing a 50% reduction in air pressure) while in the dry road position and in the slippery road position to help prevent front wheel lockup.

Limiting and quick release valves are often combined in one unit with a hand-operated control valve, usually mounted in the instrument panel. This hand-operated valve may be placed in the dry road or slipper road position to provide a 50% reduction in air pressure from the front brakes and help prevent front wheel lockup.

The limiting and quick release valves are designed to sense both the primary and secondary reservoir supply pressures, so they allow air from the secondary reservoir and control valve to actuate the rear brakes while preventing air pressure from the primary reservoir to actuate the front brakes. They are used to prevent front wheel lockup and compounded braking, especially in older air brake systems.

They also are a safety feature on some trailers and on rear axles of long wheelbase tractors to minimize delays in the application of air pressure due to the length of plumbing between the dual-control foot valve or manual trailer valve and the brakes they serve. They use air from the foot valve to send a signal to the relay valve, which then quickly directs the air from the primary and secondary reservoirs to the brakes it controls.
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