Operation CHARM: Car repair manuals for everyone.

B. Replace the HCU Actuator Piston Assemblies

1. With the ignition key in the OFF position, open the test fixture valve and then pump the brake pedal a minimum of 40 times. A noticeable change in the pedal feel will occur when the accumulators are discharged.

WARNING:
Brake system is under extreme pressure. The hydraulic control unit (HCU) may be charged with hydraulic pressure up to 2200 psi. Failure to de-pressurize the accumulators could result in personal injury and/or damage to painted surfaces. Wear safety goggles when disconnecting system fluid lines.

2. When a definite increase in pedal pressure is felt, pump the pedal a few additional times to ensure removal of all hydraulic pressure from the brake system.

3. Remove the windshield washer fluid bottle, if necessary.

4. Disconnect the two 10-way electrical connectors from the HCU and the test fixture.

5. Remove the high pressure hose fitting from the test fixture and then remove the test fixture from the HCU.

6. Disconnect the pump supply (low pressure) hose from the HCU or pump supply filter.

7. Cap all openings on brake fluid reservoir and HCU.

8. Disconnect the four brake tubes from the HCU.

9. Remove the instrument panel sight shield from below the steering column, if necessary.

10. From under the instrument panel, remove the retainer clip from the brake pedal pin. Discard the old retainer clip, a new clip must be used when the HCU is reinstalled.

11. Remove the four (4) HCU mounting nuts from the HCU studs, located under the instrument panel.

12. For AS body vehicles, disconnect the throttle position sensor (TPS) electrical connector.

13. Remove the HCU from the vehicle.

14. Remove as much brake fluid as possible from the HCU reservoir.

15. Secure the HCU by its bracket extension in a vice. Do not over tighten.

16. Remove the flange gasket from the HCU mounting bracket or the dash panel.

17. Loosen the mounting bracket from the HCU assembly enough to separate the black dust boot from the master cylinder housing, then tighten the bracket.





18. Turn the dust boot inside out and pull out of the way. Then, place a 1/2" box end wrench over a lower mounting stud (Figure 5). Place a nut on the stud and fully engage all threads. Align the wrench with the input rod bearing.

19. Push in on the wrench against the input rod bearing, moving the bearing inward approximately 1/4" to relieve the bearing load from the snap ring.

20. While holding the piston in, rotate the snap ring so one end is near the 12 o'clock position. Place a small screwdriver in the 12 o'clock keyway in the actuator bore (Figure 5). Pry the snap ring down and out of the groove with the screwdriver and pull the snap ring out of the end of the actuator.

NOTE:
It may be helpful to have a screwdriver with a bent tip to remove the snap ring.

21. Slowly release the wrench from the bearing, allowing the primary piston to pop out of the actuator. Catch any fluid from the master cylinder in a suitable container.

22. Slowly pull the primary piston assembly out of the actuator bore. Keep the assembly as straight as possible. Do not pull forcefully. If piston catches in the bore, push the piston back into the bore a short distance to align it, then try to remove it again.

NOTE:
If the piston will not come out of the bore, the actuator housing must be replaced.

23. Remove the HCU from the vice and place it, stud side down, on a lint-free bench surface.





24. Place a cloth over the outlet ports of the assembly. While holding the cloth, apply compressed air pressure (regulated to about 20 psi) to the front of the actuator to remove the secondary piston assembly from the bore (Figure 6).

25. Place the HCU back into the vice and clamp securely.

26. Using the provided long handle cleaning aid and lint-free cloth, apply clean brake fluid, which conforms to DOT 3 specifications, to the cloth and thoroughly clean the bore of the HCU. Wipe the entire circumference of the bore and work any particles toward the open end of the bore. Flush the bore, after wiping, with clean brake fluid.

27. Lubricate the cup seals, flat ring seals and 0-rings of the new piston assemblies with clean brake fluid.





28. Install the secondary piston assembly, spring end first, into the actuator. Make sure that the cup seal is not folded back or twisted. Push the secondary piston in until the rear of the piston is flush with the actuator bore opening (Figure 7).

29. Position the caged spring at the end of the primary piston assembly over the pin on the rear of the secondary piston assembly (Figure 7).

30. Slowly push both piston assemblies into the actuator bore.

CAUTION:
Do not cut, nick or twist any seals or the unit will not function properly.

31. Place the provided snap ring over the actuator push rod and bearing.

32. Push the piston assemblies all the way into the bore.





33. Place a 1/2" box end wrench over a lower mounting stud (Figure 8). Place a nut on the stud and fully engage all threads. Align the wrench with the input rod bearing.

34. Push in on the wrench against the bearing, moving the bearing inward approximately 1/4" to compress the springs, until the shoulder of the bearing is past the snap ring groove.

35. Rotate the HCU so that the bore keyway is at the top (12 o'clock) position.

36. Position the snap ring so that one end is in the 10 o'clock position and the other end is at the 8 o'clock position. The snap ring must cover the keyway.

37. Place the 10 o'clock end of the snap ring into the snap ring groove. Then, using a flat blade screwdriver, push the snap ring into the groove, working in a clockwise direction. Make sure the entire snap ring is completely seated in the groove.

38. Slowly release the wrench, and then remove the nut and wrench from the HCU lower stud.

39. Loosen the mounting bracket from the HCU. Install the rubber dust boot over the HCU bore flange and seat the retaining ring into the groove. Tighten the mounting bracket.

40. Rotate the push rod into the position shown in Figure 9, then install the HCU in the vehicle using the provided gasket.

41. Install the four HCU stud nuts and tighten to 21 ft-lbs (28 N.m).

42. Coat the surface of the brake pedal pin with lubricate or equivalent.

43. Connect the push rod to the pedal pin and install the provided retainer clip. Make sure that the brake light switch is properly adjusted.





IMPORTANT:
The HCU push rod must be assembled to the brake pedal pin as shown in Figure 9.

44. Reinstall the instrument panel sight shield, if necessary.

45. Install the four brake tubes. Tighten the fittings to 145 in-lbs (16 N.m) for AC/AY/BB vehicles or to 155 in-lbs (17 N.m) for AS vehicles.

46. For AS body vehicles, reconnect the TPS electrical connector.

47. Install the windshield washer fluid bottle, if necessary.

48. IF THE PUMP/MOTOR ASSEMBLY MUST ALSO BE REPLACED, skip to Section C. Step 9.

49. Install the pump/motor supply (low pressure) hose and tighten the clamp to 10 in-lbs (1 N.m).

50. Install the pump/motor high pressure hose and tighten to 145 in-lbs (16 N.m).

51. Connect the two 10-way HCU electrical connectors.





52. Fill the HCU brake fluid reservoir to the top of the screen on the reservoir filter/strainer with clean brake fluid conforming to DOT 3 specifications (Figure 10).

53. If the pump/motor assembly does not require replacement, continue with Section D.