The air-over-hydraulic brake system is shown in
figure 7-59. This system combines the use of
compressed air and hydraulic pressure for brake
application. Air pressure is supplied by a compressor
and stored in reservoirs as with the air brake system.
The master cylinder, wheel cylinders, and brake
construction are very similar to that used in a hydraulic
brake system. The essential difference between the
straight hydraulic brake system and the air-over-
hydraulic system lies in the AIR-HYDRAULIC-
The air-hydraulic-power cylinder (fig. 7-60) is a
self-contained power brake unit. The three essential
components of the air-hydraulic-power cylinder are as
The COMPRESSED AIR CYLINDER consists
of a large diameter air piston operating within a cylinder
body. This piston actuates a pushrod, which is attached
to the hydraulic piston within the slave cylinder.
Figure 7-60.Air-hydraulic power cylinder assembly
Movement of the piston in the compressed air cylinder
is controlled by the amount of air, under pressure, that is
allowed to enter through the control valve. The
compressed air cylinder body is attached to the end plate
on which the slave cylinder and control valve is
Figure 7-59.Air-over-hydraulic brake system.