mounted. A return spring forces the power piston to the released position when the brake pedal is released.

The SLAVE CYLINDER consists of a cylindrical housing in which a small diameter hydraulic piston operates. The outlet cap houses a residual check valve and a ball-check valve is located in the hydraulic piston.

The CONTROL VALVE consists of two poppets operating within a housing and actuated by a hydraulic relay piston and a reactionary-type diaphragm. An air control line connects the control valve to the compressed air cylinder.

AIR-OVER-HYDRAULIC BRAKE OPERATION

The air-over-hydraulic cylinder consists of an air cylinder and hydraulic cylinder in tandem, each fitted with a piston with a common piston rod between. The air piston is of greater diameter than the hydraulic piston. This difference in areas of the two pistons gives a resultant hydraulic pressure much greater than the air pressure admitted to the air cylinder. Automatic valves. actuated by fluid pressure from the master cylinder, control the air admitted to the air cylinder. Thus fluid pressure in the brake lines is always in direct ratio to foot pressure on the brake pedal.

Figure 7-61 shows the air-hydraulic power cylinder in the released position. Views A and B show the position of the valves and slave cylinder during light and heavy brake pedal application.

When the brakes are applied, as shown in view A of figure 7-61. pressure is transmitted by the brake fluid to the hydraulic piston in the slave cylinder and

the relay piston in the control valve. As hydraulic pressure builds, the relay piston moves the control valve diaphragm forward, closing the atmospheric poppet and slightly opening the air pressure poppet. Air under pressure then passes through the air control line forcing the power piston in the air cylinder forward until the air pressure on the diaphragm, in combination with spring pressure, allows the air poppet to close. The degree of brake application is determined by the amount of compressed air trapped in the power cylinder when brake pedal movement is stopped. Unless more pressure is applied or the brake pedal is released, the brakes will remain in the partially applied position.

View B of figure 7-61 shows the effect of applying high brake pedal pressure. Under this condition, the air pressure poppet is held open, allowing a full volume of compressed air to enter the air cylinder and cause full brake application.

As in a conventional hydraulic brake system the residual check valve maintains a small amount of pressure in the hydraulic system when the brakes are released. This prevents the cups in the wheel cylinders from collapsing and leaking.

REVIEW 3 QUESTIONS

Q1. What are the three essential components of an air-hydraulic-power cylinder?

Q2. In what component of the air-hydraulic-power cylinder is the residual check valve located?

Q3. What action actuates the automatic valves that control the air pressure admitted to the air cylinder?