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| Illustration 1 | g00292654 |
Parking Brake Control Valve The operator's platform is removed for clarity. | |
The parking brake control valve is located next to the accumulator charging valve on the left side of the machine. Both the parking brake control valve and the accumulator charging valve are mounted on a plate that is fastened to the top of the fuel tank.
The parking brake control valve controls the flow of pressure oil for the parking brake actuator.
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| Illustration 2 | g00292655 |
Parking Brake Control Knob (1) Parking brake control knob. | |
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| Illustration 3 | g00291888 |
Parking Brake Control Valve (2) Port for the hydraulic oil tank. (3) Port for the parking brake actuator. (4) Pressure port that is connected to the accumulator for the front service brakes (lower accumulator). | |
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| Illustration 4 | g00291889 |
View A-A (2) Port for the hydraulic oil tank. (3) Port for the parking brake actuator. (4) Pressure port that is connected to the accumulator for the front service brakes (lower accumulator). (5) Stem for the parking brake control valve. (6) Detent springs. (7) Detent balls. (8) Spool. (9) Retainer. (10) Spring. | |
When you push in on parking brake control knob (1), the parking brake disengages. This causes spool (8) and retainer (9) to move right. This movement compresses spring (10) .
This allows pressure oil to flow into the pressure port from the brake accumulator for the front service brakes. The oil then flows out of the port for the parking brake actuator (3). The oil then flows into a cavity in the actuator assembly for the parking brake.
When the oil pressure in the cavity for the parking brake actuator is high enough to compress the actuator spring, the parking brake disengages. The pressure of the oil keeps the parking brake in the DISENGAGED position.
Pull out the parking brake control knob (1) in order to engage the parking brake. Pulling out on the knob (1) causes stem (5) to move spool (8) to the left. This stops the flow of pressure oil to the parking brake actuator. This allows the oil in the cavity of the parking brake actuator to flow back to the hydraulic oil tank.
With a decrease in oil pressure for the parking brake circuit, the force of the spring in the parking brake actuator is greater than the pressure of the oil in the cavity of the parking brake actuator. This allows the spring to engage the parking brake.
If there is a decrease of hydraulic oil pressure in the service brake circuit during machine operation, the parking brake control valve will automatically engage the parking brake. If the machine is in either first speed forward or first speed reverse, the transmission will also shift into the NEUTRAL position.
With a loss of hydraulic oil pressure, the force of spring (10) moves spool (9) to the left. This blocks any flow of oil that would normally help keep spring (10) compressed. This causes spring retainer (9) and spool (8) to move left. At the same time, stem (5) also moves to the left.
Detent springs (6) and detent balls (7) move in and out of grooves that are machined into spool (8). This allows the operator to feel the internal locking mechanism of the parking brake control valve.
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| Illustration 5 | g00292657 |
Warning Category 3 Indications (11) Alert indicator for the parking brake. (12) Master action light. | |
When the oil pressure for the parking brake actuator decreases below a preset value, the Caterpillar Monitoring System initiates a Warning Category 3 alarm. The indications of a Warning Category 3 include the following items: the parking brake indicator light (11), a flashing master action light (12) on the Caterpillar Monitoring System and the action alarm.
The transmission returns to the NEUTRAL position if the transmission is in either first speed forward or first speed reverse. This will occur when the parking brake engages.
Note: The parking brake can be manually engaged and disengaged when the pressure in the system is above 10300 kPa (1495 psi). However, if the system pressure is below 10300 kPa (1495 psi), actuation force increases.
Note: If the parking brake is in the ENGAGED position, the parking brake control valve will move to the ENGAGED position when the pressure difference between the pressure port and the tank port is approximately 2400 kPa (350 psi).
Note: Hydraulic tank pressure above 410 kPa (60 psi) may cause stem (5) to extend to the ENGAGED position.
Reference: For additional information on the parking brake control valve, refer to Service Manual, "Brake System - Testing and Adjusting" for the machine that is being serviced.