Hydraulic system - bleeding

Braking system / Hydraulic system - bleeding

1 If the master cylinder or the pressure regulating valve has been disconnected and reconnected then the complete system (both circuits) must be bled.

2 If a component of one circuit has been disturbed then only that particular circuit need be bled.

3 The two disc brakes comprise the front circuit and the two rear brakes the rear circuit.

4 Unless the pressure bleeding method is being used, do not forget to keep the fluid level in the master cylinder reservoir topped up to prevent air from being drawn into the system which would make any work done worthless.

5 Before commencing operations, check that all system hoses and pipes are in good condition with all unions tight and free from leaks.

6 Take great care not to allow hydraulic fluid to come into contact with the vehicle paintwork as it is an effective paint stripper.

Wash off any spilled fluid immediately with cold water.

7 As the system on 55 and 70 models incorporates a vacuum servo, destroy the vacuum by giving several applications of the brake pedal in quick succession. The car should be loaded with enough weight to actuate the pressure regulating valve before bleeding commences.

Bleeding - two man method 8 Gather together a clean glass jar and a length of rubber or plastic tubing which will be a tight fit on the brake bleed screws (photo).

12.8 Caliper bleed screw with dust cap fitted

9 Engage the help of an assistant.

10 Push one end of the bleed tube onto the flrst bleed screw and immerse the other end of the glass jar which should contain enough hydraulic fluid to cover the end of the tube.

11 Open the bleed screw one half a turn and have your assistant depress the brake pedal fully then slowly release it. Tighten the bleed screw at the end of each pedal downstroke to obviate any chance of air or fluid being drawn back into the system.

12 Repeat this operation until clean hydraulic fluid, free from air bubbles, can be seen coming through into the jar.

13 Tighten the bleed screw at the end of a pedal downstroke and remove the bleed tube.

Bleed the remaining screws in a similar way.

Bleeding - using a one way valve kit
14 There are a number of one-man, one-way brake bleeding kits available from motor accessory shops. It is recommended that one of these kits is used wherever possible as it will greatly simplify the bleeding operation and also reduce the risk of air or fluid being drawn back into the system quite apart from being able to do the work without the help of an assistant.

15 To use the kit, connect the tube to the bleedscrew and open the screw one half a turn.

16 Depress the brake pedal fully and slowly release it. The one-way valve in the kit will prevent expelled air from returning at the end of each pedal downstroke. Repeat this operation several times to be sure of ejecting all air from the system. Some kits include a translucent container which can be positioned so that the air bubbles can actually be seen being ejected from the system.

17 Tighten the bleed screw, remove the tube and repeat the operations on the remaining brakes.

18 On completion, depress the brake pedal. If it still feels spongy repeat the bleeding operations as air must still be trapped in the system.

Bleeding - using a pressure bleeding kit
19 These kits too are available from motor accessory shops and are usually operated by air pressure from the spare tyre.

20 By connecting a pressurised container to the master cylinder fluid reservoir, bleeding is then carried out by simply opening each bleed screw in turn and allowing the fluid to run out, rather like turning on a tap, until no air is visible in the expelled fluid.

21 By using this method, the large reserve of hydraulic fluid provides a safeguard against air being drawn into the master cylinder during bleeding which often occurs if the fluid level in the reservoir is not maintained.

22 Pressure bleeding is particularly effective when bleeding “difficult” systems or when bleeding the complete system at time of routine fluid renewal.

All methods
23 When bleeding is completed, check and top up the fluid level in the master cylinder reservoir.

24 Check the feel of the brake pedal. If it feels at all spongy, air must still be present in the system and further bleeding is indicated.

Failure to bleed satisfactorily after a reasonable period of the bleeding operation, may be due to worn master cylinder seals.

25 Discard brake fluid which has been expelled. lt is almost certain to be contaminated with moisture, air and dirt making it unsuitable for further use. Clean fluid should always be stored in an airtight container as it absorbs moisture readily (hygroscopic) which lowers its boiling point and could affect braking performance under severe conditions.

Fig. 8.12 Bleeding a rear wheel cylinder (Sec 12)