When Sequence Control starts, the left front solenoid cycles first, then the right front solenoid and finally the right rear. As the right rear solenoid works, pressure in the left rear brake circuit is simultaneously regulated by the mechanical valve. It is not necessary to exactly match the values in the pressure table during your tests. The important thing is that you observe the sequence shown in the table below:
The pressure check is repeated for each of the four HCU pressure output ports. Recall that three of the brake hydraulic circuits are controlled by solenoid valves (all but the left rear circuit). If a check of any one of these circuits yields incorrect pressure readings, this may indicate a non-functioning solenoid valve.
• Intitial Value: the hydraulic circuit pressurizes as you press the brake pedal.
• Decompressed: the circuit automatically loses pressure as the HCU cycles the solenoid valve to pressure-reduce mode.
• Compressed: the circuit regains pressure (at least equal to the initial reading) as the system returns to pressure-increase mode.
Normal Pressure Readings:
Circuit Initial Value Decompressed Compressed Front Brakes 498 psi (3432 kPa) 71 psi (490 kPa) 498 psi (3432 kPa)
Rear Brakes 498 psi (3432 kPa) 71 psi (490 kPa) 498 psi (3432 kPa)
Typical incorrect readings are shown in the two tables below. In the first example, the pressure does not decrease from the initial value. In the second example, the pressure fails to reach the initial value at the start of Sequence Control.
Caution: For this check, use a pressure gauge that is reserved exclusively for brake fluid. Do NOT use a gauge that has been used for transmission pressure tests; doing so could lead to brake system malfunctions.
Note: Before you start the HCU pressure checks, pump the brake pedal several times. That will bleed vacuum from the vacuum booster so that it is at atmospheric pressure.
Note: This check requires that you press on the brake pedal so that the pressure gauge displays a pressure equal to or more than the initial value in the table. Be prepared to press on the brake pedal with considerable effort for the duration of the check.
To perform an HCU pressure check, follow these steps:
• At the HCU, loosen the hydraulic fluid line for the left front brake circuit.
• In its place, attach a suitable pressure gauge.
• Bleed air from the pressure gauge.
• Activate the Sequence Control mode.
• Observe the pressure gauge as the system cycles the left front solenoid valve through pressure reduce, pressure hold, and pressure increase. Compare the readings to the values in the table.
• Move the pressure gauge to the right front brake circuit connection at the HCU and reconnect the previously disconnected pipe. Then repeat Steps 3 through 5.
• Repeat these steps for the two remaining HCU ports (left and right rear).
When you have finished these checks, make sure all of the brake fluid pipe
connections are secure and free from leaks and that the system is bled properly.