Hydraulic system - bleeding (anti-lock braking system)

Note: Before starting work, refer to the warning at the beginning of Section 11 concerning the dangers of hydraulic fluid.

1 On cars equipped with the anti-lock braking system there are two bleed procedures possible according to which part of the hydraulic system has been disconnected.

2 If any one of the following conditions are present, bleed procedure A should be adopted:
a) A modulator has been removed.

b A modulator-to-master cylinder return hose has been drained.

c) The two modulator hydraulic hoses have been removed.

3 If any one of the following conditions are present, bleed procedure B should be adopted:
a) Any condition where the master cylinder has been drained providing that the modulator fluid return pipe has not lost its head of fluid.

b) Removal of any of the basic braking system components ie brake caliper, flexible hose or pipe, wheel cylinder, load apportioning valve.

Bleed procedure A
4 Top-up the master cylinder reservoir to the “MAX” mark using the specified type of fluid and keep it topped up throughout the bleed procedure.

5 Using a Torx type key or socket bit slacken the bypass valve on the relevant modulator by one to one and a half turns. The bypass valve is located between the two flexible hoses on the side of the modulator (see illustration).

23.5 Modulator bypass valve (A) location
23.5 Modulator bypass valve (A) location

6 Fully depress and hold depressed the auto bleed plunger on the modulator so that the plunger circlip contacts the modulator body (see illustration).

23.6 Modulator auto bleed plunger location (arrowed)
23.6 Modulator auto bleed plunger location (arrowed)

7 Have an assistant steadily pump the brake pedal at least twenty times while you observe the fluid returning to the master cylinder reservoir. Continue this operation until the returning fluid is free from air bubbles.

8 Release the auto bleed plunger ensuring that it has fully returned. Pull it out by hand if necessary.

9 Tighten the bypass valve on the modulator.

10 Now carry out bleed procedure B.

Bleed procedure B
11 This procedure is the same as for conventional braking systems and reference should be made to Section 11. Note, however, that all the weight of the car must be on the roadwheels, not suspended wheel free, otherwise the load apportioning valves will not bleed.

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