On vehicles equipped with air-over-hydraulic
brakes (fig. 6-12), do a good visual inspection of the air
compressor, the air reservoir, the air lines, the brake
pedal and linkage, the wheel brakes, the master cylinder,
and the hydraulic line from the master cylinder to the
air-hydraulic-power cylinder and from the air-hydraulic
power cylinder to the wheel brakes.
Operating troubles resulting from malfunction of
the air-over-hydraulic power cylinder are hard pedal
(excessive pedal pressure required to apply the brakes)
and dragging brakes (power cylinder fails to return to
released position when the brake pedal is released).
To test a sluggish or inoperative power cylinder, first
install an air pressure gauge in the control valve housing
and a hydraulic gauge at both the hydraulic fluid inlet
line and the hydraulic brake line output port. Then
slowly depress the brake pedal and observe the gauges.
When the air control pressure gauge shows between 1
and 5 psi, the hydraulic pressure at the hydraulic inlet
should not exceed 40 psi. Excessive hydraulic pressure
indicates a sticking relay piston (caused by swollen or
damaged piston scaling cups or a corroded or damaged
relay piston sleeve) or sticking control valve poppets
(caused by corrosion of the poppets, poppet seats, or
With the brake pedal completely depressed in the
fully applied position, the air control pressure gauge
should show 90 psi and the hydraulic output pressure
gauge should show full power (runout) pressure of 1,400
to 1,600 psi. Low pressure or no pressure on the air
pressure gauge indicates air leakage or an inoperative
control valve. Low hydraulic output pressure indicates
hydraulic fluid leakage, a sticking hydraulic piston, or
an inoperative check valve (in the hydraulic piston), or
a residual line check valve.
To test for internal and external air leakage or
hydraulic leakage, fast depress the brake pedal and
apply soapsuds at the air control line and its connections,
the double check valve (if so equipped), and the cylinder
body and end plate. Bubbles appearing at any of these
points indicate external air leaks. While the pedal is
depressed, check for hydraulic fluid leakage at the outlet
fitting cap and around the jam nut on the slave cylinder
housing. Internal air leakage is indicated by a pressure
drop in excess of 2 psi in 15 seconds. The trouble is a
worn or damaged piston packing, a scored cylinder
body, or leakage at the poppets in the control valve.
Internal hydraulic pressure leakage can also be indicated
by hydraulic pressure drop at both hydraulic pressure
gauges while the brake pedal is depressed.
Dragging brakes can be tested by releasing the brake
pedal and observing the air pressure gauge and the two
Figure 6-12.Air-over-hydraulic brake system.