TM 5-3805-291-23-1
THEORY OF OPERATION - CONTINUED
0003 00
BRAKE AND HYDRAULIC FAN SYSTEM - CONTINUED
(3)
Push bottom of parking brake switch to disengage parking brake (TM 5-3805-291-10). This causes spool
and retainer to move right. This movement compresses spring.
(4)
This allows pressure oil to flow into pressure port from rear service brakes brake accumulator. Oil then
flows out of parking brake actuator port and into a cavity in parking brake actuator assembly.
(5)
When oil pressure in parking brake actuator port cavity is high enough to compress actuator spring, park-
ing brake disengages. Oil pressure keeps parking brake in DISENGAGED position.
(6)
Push top of parking brake switch to engage parking brake (TM 5-3805-291-10). This causes stem to move
spool left. This stops pressure oil flow to parking brake actuator, allowing oil in parking brake actuator
cavity to flow back to hydraulic oil tank.
(7)
With a decrease in oil pressure for parking brake circuit, spring force in parking brake actuator is greater
than oil pressure in parking brake actuator cavity. This allows spring to engage parking brake.
(8)
If there is a decrease in hydraulic oil pressure for service brake circuit during machine operation, parking
brake control valve will automatically engage parking brake. If machine is in either first speed forward or
reverse, transmission will also shift to NEUTRAL.
(9)
With a loss of hydraulic oil pressure, spring force moves spool left. This blocks any flow of oil that would
normally help keep spring compressed, and causes spring retainer, spool, and stem to move left.
(10)
Detent springs and detent balls move in and out of grooves machined into spool. This allows an operator to
feel internal locking mechanism of parking brake control valve.
N OT E
Parking brake can be manually engaged or disengaged when system pressure is above 1,495 psi (10,300
kPa). However, if system pressure is below 1,495 psi (10,300 kPa), actuation force increases.
If parking brake is DISENGAGED, parking brake control valve will move to ENGAGED position
when pressure difference between pressure port and tank port is approximately 350 psi (2,400 kPa).
Hydraulic tank pressure above 100 psi (700 kPa) may cause stem to extend to ENGAGED position.
(11)
When parking brake actuator oil pressure decreases below a preset value, transmission returns to NEU-
TRAL if in either first speed forward or reverse. Parking brake will automatically engage when this
occurs. An alert indicator for brakes will flash and an alarm will sound (TM 5-3805-291-10).
h.
Parking Brake.
427-C1771
0003 00-95
