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TM 9-8000
Section IX. 34-40.Construction. The air-over-hydraulic brake
34-41. Operation. The air-hydraulic cylinder (fig. 34-40)
system is shown in figure 34-40. Air pressure is supplied
consists of an air cylinder and a hydraulic cylinder in
by a compressor and stored in reservoirs, as with the
tandem, each fitted with a piston with a common piston
airbrake system. The master cylinder is similar to the
rod between. The air piston is of greater diameter than
master cylinders used in hydraulic brakes. Also, the
the hydraulic piston. This difference in areas of the two
wheel brake cylinders and wheel brake construction are
pistons gives a resultant hydraulic pressure much
very similar to that used in hydraulic brakes. The
greater than the air pressure admitted to the air cylinder.
essential difference between the straight hydraulic brake
Automatic valves, actuated by fluid pressure from master
system and the air-over-hydraulic brake system lies in
cylinder, control the air admitted to the air cylinder. Thus,
the air-hydraulic cylinder. This cylinder consists of three
the fluid pressure in brake lines is always in a direct ratio
essentials: a large-diameter air piston; a small-diameter
to foot pressure on brake pedal. An air line from an air-
hydraulic piston in tandem with it, both on the same rod;
hydraulic cylinder leads to a trailer coupling at the rear of
and a set of valves controlled by hydraulic pressure from
the master cylinder for admitting air into the air-cylinder
section of the air-hydraulic cylinder.
Valve action varies with the amount of brake pedal
pressure, as mentioned in paragraph 34-39d. When
heavy brake pedal pressure Is applied by the
Figure 34-40. Air-Over-Hydraulic Brake System.


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