SECONDARY BRAKING AND
AUTOMATIC APPLY
A fundamental function of the secondary brake system
is to provide reserve braking in the event of any single
failure. For this reason, the system is divided into
multiple circuits, each with its own isolation check
valve, two accumulators, and circuit regulator. The
secondary system becomes whatever circuit(s) is op-
erable after a failure. If the failure is a jammed treadle
valve, then the brake lock becomes the secondary
system, otherwise, either of the two brake circuits
would be the secondary system.
The four brake accumulators perform two functions; to
provide rapid flow for good response and to store
energy for secondary braking. The check valves assure
this energy is retained should a failure occur in the
brake system supply or an accumulator circuit. An
additional check valve located between the supply line
from the brake/steering pump and the brake manifold,
provides additional protection against pressure loss if
the oil supply is interrupted.
If a failure occurs in the pump, steering or either brake
accumulator circuit, a low brake pressure warning light
(on the overhead display panel in the cab) and an
audible alarm will actuate and the vehicle should be
stopped as soon as practical. When the pressure in one
accumulator circuit is less than the preset level, all the
service brakes will be automatically applied. Automatic
brake application is accomplished by the “Automatic
Apply Valve” (PS), located in the brake manifold. This
valve senses the lower brake accumulator pressure,
and when the pressure is less than 1650 psi (11.4 MPa),
the valve shifts, operating the brake treadle valve hy-
draulically which in turn applies pressure to the dual
relay valves and applying all the brakes.
Regardless of the nature of location of a failure, sensing
the lowest brake accumulator circuit pressure assures
two to four full brake applications after the low brake
warning light and buzzer, and before automatic apply.
This allows the operator the opportunity to safely stop
the truck after the warning has turned on.
PARKING BRAKE CIRCUIT
The parking brakes are spring applied and hydrauli-
cally released.
NOTE: Whenever the park brake solenoid is de-ener-
gized, a spring in the solenoid valve will shift the spool,
diverting oil pressure from the parking brakes to direct
the oil back to the hydraulic tank.
Normal Operation (key switch on, engine running)
Parking brake switch “ON”
The parking brake solenoid (15, Figure 2-1) is
de-energized. The oil pressure in the parking
brake lines return to tank and the springs in the
parking brake will apply the brake. The parking
brake pressure switch (21) will close, completing
a path to ground, and illuminating the parking
brake light on the overhead display panel.
Parking brake switch “OFF”
The parking brake solenoid is energized. The oil
flow is routed from the park brake solenoid, to the
park brake calipers for release. The parking brake
circuit is protected against accidental application
by monitoring a wheel motor speed sensor to
determine truck ground speed. The park brake
will not apply until the truck is virtually stopped.
This eliminates park brake damage and will ex-
tend brake adjustment intervals.
If the key switch is turned “OFF” (park brake
switch “ON” or “OFF”), the park brake will not
apply until vehicle speed is less than 1/3 MPH (0.5
km/h).
If a loss of hydraulic supply pressure occurs, with
the parking brake switch “OFF”, the parking
brake solenoid will still be energized. The supply
circuit (that lost pressure) is still open to the
parking brake calipers. To prevent park brake
pressure oil from returning to the supply circuit,
a check valve (in the park brake circuit) traps the
oil, holding the parking brake in the released
position.
NOTE: Normal internal leakage in the parking brake
solenoid may allow leakage of the trapped oil to return
back to tank, and eventually allow park brake applica-
tion.
If 24 volt power to the solenoid is interrupted, the
park brake will apply at any vehicle speed. The
spring in the solenoid will cause it to shift, opening
a path for the oil pressure in the park brake line
to return to tank and the springs in the parking
brake will apply the brake. The parking brake
pressure switch (21) will close, completing a path
to ground, illuminating the parking brake light on
the overhead display panel and interrupting pro-
pulsion.
J02027 Brake Circuit J2-3
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