Secondary Braking And Automatic Apply; Parking Brake Circuit - Komatsu 830E Shop Manual

SECONDARY BRAKING AND
AUTOMATIC APPLY
A fundamental function of the 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, accumulator, and circuit regulator. The secon-
dary system becomes whatever circuit(s) is operable
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 brake accumulators (1 & 3, Figure 2-1), (as de-
scribed under service brake circuit) perform two func-
tions; rapid flow for good response and store energy
for secondary braking. The circuit check valve assures
this energy is retained should a failure occur in brake
system supply or an accumulator circuit.
If a failure occurs in the pump, steering or either brake
accumulator circuit, a low brake pressure warning light
(on the instrument panel) and an audible alarm (in the
cab) will actuate and the vehicle should be stopped as
soon as practical. When the pressure in one accumu-
lator 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" (PS1, 19), located in the brake manifold. This
valve senses the lower brake accumulator pressure,
and when the pressure is less than 2000 ±75 psi
(13800 ±520 kPa), the valve shifts, operating the brake
treadle valve and applying all the brakes full on.
Regardless of the nature of location of a failure, sens-
ing the lowest brake accumulator circuit pressure as-
sures 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 brake is spring applied and hydraulically
released.
NOTE: Whenever the park brake solenoid is de-ener-
gized, a spring in the solenoid valve will shift the spool
to the position to allow the park brake to be applied.
J02032
Normal Operation (key switch on, engine running)
• Parking brake switch "ON"
The parking brake solenoid (16) 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 (23) will close, completing a path to
ground, and illuminating the parking brake light
on the instrument panel.
• Parking brake switch "OFF"
The parking brake solenoid is energized. The
pressure oil is routed from the park brake sole-
noid, to the parking brake pressue regulator (4)
(reducer), then to the park brake calipers for
release. The parking brake circuit is protected
against accidental apply by monitoring a wheel
motor speed sensor. The park brake with not
apply until the truck is virtually stopped. This
eliminates park brake damage and will extend
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,
due to the monitoring of the wheel motor speed.
• If loss of hydraulic supply pressure occurs, with
Parking brake switch "OFF", the parking brake
solenoid will still be energized. The hydraulic
supply circuit is still open to the parking brake
calipers. A check valve in the park brake hydraulic
supply circuit traps the oil, holding the parking
brake in the release position.
NOTE: Normal internal leakage in the parking brake
solenoid and the pressure reducing valve may allow
leakage of the trapped oil to return back to tank, and
eventually allow park brake application.
• If 24 volt power to the solenoid is interupted, 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 (23) will close, completing a path
to ground, and illuminating the parking brake light
on the instrument panel.
Brake Circuit J2-3