Figure 3-46.—Hand brake valve.
Figure 3-48.-Air-over-hydraulic power cylinder.
NOTE: More information about the air brake
system is in the chapter covering tractor and trailer
operations.
AIR-OVER-HYDRAULIC BRAKE SYSTEM
An air-over-hydraulic brake system is shown in
figure 3-47. This system combines the use of
compressed air and hydraulic pressure for brake
operation. The air-over-hydraulic brake system has an
trailer brakes without applying the truck or tractor
air-over-hydraulic power cylinder (fig. 3-48) that
brakes. The hand brake valve or independent trailer
contains an air cylinder and a hydraulic cylinder in
control valve, as shown in figures 3-44 and 3-46,
tandem. Each cylinder is fitted with a piston and a
provides the operator control of the trailing load at all
common rod. The air piston is of greater diameter than
times.
the hydraulic piston. This difference in the two pistons
Figure 3-47.-Typical air-over-hydraulic brake system.
3-26
