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.

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.

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

Figure 7-61.- Air-hydraulic power cylinder (Air-Pak) during operation.