Lesson 2/Learning Event 2
A
Doublecheck valve assembly
N
Brake pedal
B
Air control line
P
Residual line check
C
Relay piston
valve assembly
D
Diaphragm assembly
Q
Hydraulic piston
E
Exhaust port
R
Poppet return spring
F
Trailer connection
S
Diaphragm return spring
G
Atmospheric poppet closed
T
H
Air pressure poppet open
U
Hydraulic inlet line
J
Compressed air reservoir
V
Pushrod
K
Air compressor
W
Piston return spring
L
Compressed air inlet line
X
Power piston
M
Hydraulic outlet port
Y
Trailer connection
When the brake pedal is pushed down, brake fluid is pushed through
the hydraulic system. The fluid pressure is transmitted from the
slave cylinder to the relay piston in the control unit. The relay
piston changes the hydraulic pressure to mechanical motion, which
pushes the diaphragm assembly against the atmospheric poppet to close
the air exhaust passage and to open the air pressure poppet.
Compressed air can now flow from the reservoir, through the
compressed air line, and past the open air pressure poppet in the
control unit. The airflow is directed into two control lines
leaving the control unit. One line is to the service outlet to
control the trailer brakes. The other control line carries the air
pressure to the power cylinder of the airhydraulic cylinder. The
air pressure forces the power piston and the attached pushrod toward
the slave cylinder.
The extending pushrod contacts the slave cylinder hydraulic piston
and pushes it forward. This closes the check valve in the hydraulic
piston and brake fluid under pressure is forced through the outlet to
the wheel cylinder to apply the brakes. The fluid pressure to the
wheel brakes is now being produced from two sources. The air
pressure that is pushing the power piston ahead is one source. The
second source is from the pressure applied on the brake pedal.
Hydraulic pressure from the master cylinder pushes on the slave
cylinder piston, along with the pushrod, so the two forces add
together.
The amount of air pressure permitted to enter the power cylinder is
regulated by the pressure applied to the brake pedal. To see how
this is possible, assume that the driver applies enough pressure on
the brake pedal to slow the truck down but not enough to lock the
wheels.
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