3.2 Brake Chambers, Control Valves, Quick Release Valves, and Relay Valves

Brake Chambers, Control Valves, Quick Release Valves, and Relay Valves

Quick Answer: A brake chamber turns air pressure into pushrod force: force = pressure × diaphragm area. A Type 30 chamber (30 square inches) at 60 PSI pushes about 1,800 lb. The vehicle manufacturer chooses the chamber size, and the driver controls the pressure. Control valves have an inlet valve and an exhaust valve that apply (inlet open, exhaust closed), hold (both closed), or release (exhaust open). Quick release valves let chamber air escape near the chambers. Relay valves near the axles use a small control signal to deliver large volumes of tank air, and their crack pressures time the application along the vehicle.

Every service circuit works the same way

The Alberta course first builds one simplified service circuit, then applies it to the steering axle, the drive axles, and each trailer axle group. The basic circuit is a tank, a control valve, and brake chambers. Quick release and relay valves are variations that make the circuit faster.

Brake chambers: air pressure into mechanical force

A brake chamber is the end of the air system and the start of the mechanical linkage. Air enters behind a flexible diaphragm, which pushes a push plate and pushrod out toward the slack adjuster. Two factors decide the force:

1. Chamber size (diaphragm area). The manufacturer chooses it.
2. Air pressure applied. The driver controls it with the pedal.

Pressure is force per unit area, so force = pressure × area:

Typical sizes: Type 20 (20 sq in, 129 sq cm) on steering axles and Type 30 (30 sq in, 194 sq cm) on load-bearing axles, meaning the drives and trailers.

Long-stroke chambers have more total stroke before the push plate bottoms out. A standard Type 30 has a total stroke of 2½ inches (64 mm), and a long-stroke Type 30 has 3 inches (77 mm). Long-stroke chambers are identified by square inlet ports and are not interchangeable with standard chambers. Section 6.2 gives the adjustment limits for each.

Control valves: apply, hold, release

All control valves work the same way, whether they are operated by the driver's foot or hand, by an air signal, or by an electric signal. Inside are two valves: an inlet valve, which lets tank air flow to the chambers, and an exhaust valve, which lets chamber air escape to the atmosphere.

When you apply the brakes, tank pressure and the gauge reading drop, because some air has moved into the chambers. No air volume is lost from the system at that moment. The air is lost to the atmosphere only when the brakes are released. That is why each release, not each application, costs you reserve air, and why pumping the brakes drains the tanks.

Quick release valves

If chamber air had to travel all the way back to the foot valve to exhaust, release would be slow. A quick release valve mounted near the chambers lets application air escape right there when the control pressure drops. Air may also exhaust through the antilock brake system (ABS) modulators if the vehicle has them. Quick release valves exhaust forcefully, so keep clear of them when someone releases the brakes.

Relay valves: beating brake lag

On a long vehicle, air sent from the tank to the foot valve at the cab and then back along the chassis to the drive-axle or trailer chambers would take a long time to arrive. A relay valve solves this:

• The relay is mounted near the axle group. It is often mounted in the service tank on trailers and on the frame for drive axles. It has its own large, direct supply from the tank.
• The foot valve sends only a small control (pilot) signal to the top of the relay. The relay then opens and delivers tank air directly to the chambers.
• On release, the chamber air exhausts from under the relay, and only the small volume in the control line exhausts back through the foot valve.
• Today's vehicles may have electronic brake management systems that pilot relays electrically, but all vehicles must keep air pressure control signals.

Line sizes follow the job. Large volumes move through larger hoses, while signals travel faster in smaller lines:

Crack pressure: timing the brakes along the vehicle

Crack pressure is the control pressure that must reach the top of a relay before it starts to open and send tank air to the chambers. The control signal comes from the cab, so relays farther back get it later. To time the application along the vehicle, relays closer to the cab have higher crack pressures. This gives the signal time to reach the rear before the power unit's brakes apply.

The exception is the service line booster relay. Long trailer service lines weaken the signal (transmission loss), so a special booster relay re-creates it. A booster relay has zero crack pressure, because any crack pressure would reduce the signal below what the tractor's foot valve sent. Booster relays are often found on converter dollies. Relays with different crack pressures look alike, so failed valves must be replaced with the correct part.