Air Brake Service System

Brake application valve

The foot operated brake application valve supplies air pressure from the reservoirs to the brake chambers at each wheel when the brakes are applied. The air pressure supplied through the brake application valve is proportional to valve movement.

The brake application valve may be floor-mounted, or mounted on the firewall with a suspended pedal. Some of the brake application valves are mounted horizontally on the firewall and operated by a pushrod from the brake pedal. Many floor-mounted or firewall-mounted brake application valves are mounted vertically and operated by a roller and plunger from the brake pedal or treadle.

Since the standards require dual brake systems with separate reservoirs for the front and rear brakes, dual brake application valves must be used in the brake system.

When the brakes are applied (the driver steps on the brake pedal or treadle) the dual brake valve delivers air pressure from the primary ports to the rear brakes and from the secondary delivery ports to the front brakes.

When the brake pedal or treadle is released, the primary and secondary exhaust valves are open and air in the primary and secondary delivery lines is exhausted to the air.

Quick Release Valve (Rapid-exhaust valve)

Allows air to flow through the valve when the brakes are applied, but this valve released air pressure quickly when the brakes are released. A quick released valve is connected in many front-wheel brake systems, and this valve may be used on rear brakes.

Since the quick release valve is mounted closer to the front axle brake chambers than the brake application valve, air can be exhausted faster from the quick release valve than from the brake application valve. Quick brake release is necessary to reduce brake lining wear and drum overheating.

Service Brake Relay Valves

Full air pressure is supplied from the primary (rear axle) reservoir to the service brake relay valve supply port. During a brake application, air pressure is supplied from the brake application valve to the service brake relay valve control port. When this air pressure is supplied to the control port, the valve supplies air pressure from the supply port to rear wheel brake chambers.

Since the rear axle service reservoir is located closer to the rear brake chambers than the brake application valve, air pressure can be supplied faster from the rear axle reservoir through the service brake relay valve than it can be supplied through the brake application valve. The service brake relay valve is usually mounted on or near the rear axle. The service brake relay valve is usually used on long wheelbase trucks.

On a tandem axle truck, they used two relay valves or one relay valve with four outlet ports.

The air pressure above the relay valve piston that is required to open the inlet valve is called the crack pressure Service brake relay valves available with different crack pressures form 4 to 10 psi (27.58 kPa to 68.95 kPa).

Limiting Valve

The purpose of the limiting valve is to reduce the air pressure supplied to the front brakes during moderate brake applications. Because there is usually more loads on the rear wheels of a truck compared to the front wheels, the front wheels have more tendency to lock up during a brake application, arid the limiting valve reduces this tendency.

The limiting valve is connected between the brake application valve and the quick release valve.

Limiting Quick Release Valve

In some older air brake systems the limiting and quick release valves are combined in one unit. A hand-operated control valve mounted in the instrument panel is used with the limiting, quick release valve. The control valve may be placed in the dry road or slipper road position. The limiting quick release valve reduces air pressure to the front brakes (provide a 50% reduction in air pressure) with the control valve in the slippery road position to help prevent front wheel lockup.

Pressure Protection Valves

This valve connected in the air line from the air brake system to air-operated accessories such as a driver’s air seat or air starter.

Some pressure protection valves are externally adjustable, and others have a fixed pressure setting. Both of these valves are available with various pressure ratings. The pressure protection valve protects the air brake system from loss of air pressure if a leak occurs in an air-operated device such as an air starter, or air seat.

Pressure Reducing Valves

The pressure reducing valves are used to supply air pressure to air operated accessories such as an air seat that requires a lower pressure than the brake system.

Control Valves

TW-4 is a manually operated control valve that supplies air pressure to the AS-1 control valve. When air pressure is supplied from the TW-4 to the AS-1, the AS-1 opens and supplies air pressure from the auxiliary reservoir to the air starter.

Inversion Valves and Safety Actuator Design

Inversion valves and safety actuators or brake chambers are used on some transit and inter-city buses. The inversion valve is connected in the lines between the system park valve and the paring brake relay valve. The inversion valve is normally open and allows air pressure to flow from the supply port to the delivery port.

The inversion valve may be used with double diaphragm (DD) safety actuators. A safety actuator has three ports: a lock port, service port, and emergency port. Air pressure supplied through the service port is applied to the service diaphragm, and air pressure supplied through the emergency port is applied to the auxiliary diaphragm. The service diaphragm is larger than the auxiliary diaphragm. When the air pressure is supplied to the lock port, the rollers are disengaged and this allows the actuator shaft to move. If air pressure is released from the lock port, the rollers lock the plunger shaft.

While driving the truck, the system park valve is pushed inward and air pressure is supplied through this valve to the inversion valve and the lock ports on the safety actuators. The air pressure supplied to the lock ports on the safety actuators releases the rollers so the actuator shaft is unlocked. The air pressure supplied to the inversion valve closes this valve, and shuts off the air pressure to the parking brake relay valve. Under this condition the parking brake relay valve remains closed and no air pressure is supplied to the auxiliary actuator diaphragm.

When the service brakes are applied, air pressure is supplied from the service brake relay valve to the service port on the safety actuators. This air pressure moves the service diaphragm and actuator shaft to apply the service brakes.

If the driver pulls the system park valve outward to apply the parking brakes, the air pressure from the actuator lock ports and the inversion valve is exhausted thorough the system park valve. Under this condition the inversion valve opens and air pressure is supplied from the reservoir to the parking brake relay valve. This air pressure opens the parking relay valve and air pressure is supplied from the reservoir through the parking brake relay valve to the auxiliary ports on the safety actuators. This air pressure supplied through the auxiliary ports to the auxiliary diaphragms moves this diaphragm and the actuator shaft to apply the parking brakes. If the air pressure applying the parking brakes leaks, the actuator shaft retracts lightly and is then locked by the wedging action of the rollers to keep the parking brakes applied.

The rollers and parking brakes are released when the system park valve is pushed inward and air pressure above 40 psi (275.8 kPa) is supplied through this valve to the actuator lock ports. To release the rollers and parking brakes it is also necessary to apply the service brakes with air pressure equal to or above the pressure that was supplied to the auxiliary diaphragms.

Compressed-air cylinders (actuators)

Check valves

Single check valves

Allow airflow in one direction and block airflow in the opposite direction (air flows though the valve in one direction). One of the most common locations for one-way valve is between the supply reservoir and the primary and secondary reservoirs.

Double check valve

Have two inlet ports and one outlet port. A double check valve allows air to flow from the inlet with the highest pressure to the outlet. The supply ports on double check valve are connected to the rear axle and front axle service reservoirs, and the delivery port of the check valve is connected to the parking and emergency brake system.