Hydraulic system - bleeding
Note: Refer to Warning 1 in Section 1 before starting work.
1 When a maintenance operation has only affected one circuit of the hydraulic system, then bleeding will normally only be required to that circuit (front and rear diagonally opposite). If the master cylinder or the pressure regulating valve have been disconnected and reconnected, then the complete system must be bled.
2 One of three methods can be used to bleed the system.
Bleeding - two-man method
3 Gather together a clean jar and a length of rubber or plastic bleed tubing which will fit the bleed screw tightly. The help of an assistant will be required.
4 Take great care not to spill onto the paintwork as it will act as a paint stripper. If any is spilled, wash it off at once with cold water.
5 Clean around the bleed screw on the front right-hand caliper and attach the bleed tube to the screw.
Bleed tube attached to bleed screw on front brake
6 Check that the master cylinder reservoir is topped up and then destroy the vacuum in the brake servo (where fitted) by giving several applications of the brake foot pedal.
7 Immerse the open end of the bleed tube in the jar, which should contain 50 to 76 mm of hydraulic fluid. The jar should be positioned about 300 mm above the bleed nipple to prevent any possibility of air entering the system down the threads of the bleed screw when it is slackened.
8 Open the bleed screw half a turn and have your assistant depress the brake pedal slowly to the floor and then quickly remove his foot to allow the pedal to return unimpeded. Tighten the bleed screw at the end of each downstroke to prevent expelled air and fluid being drawn back into the system.
9 Observe the submerged end of the tube in the jar. When air bubbles cease to appear, fully tighten the bleed screw when the pedal is being held down by your assistant.
10 Top-up the fluid reservoir. It must be kept topped up throughout the bleeding operations. If the connecting holes in the master cylinder are exposed at any time due to low fluid level, then air will be drawn into the system and work will have to start all over again.
11 Repeat the operations on the left-hand rear brake,
Rear brake bleed screw and protective cap
the left-hand front and the right-hand rear brake in that order (assuming that the whole system is being bled).
12 On completion, remove the bleed tube. Discard the fluid which has been bled from the system unless it is required for bleed jar purposes, never use it for filling the system.
Bleeding - with one-way valve
13 There are a number of one-man brake bleeding kits currently available from motor accessory shops. It is recommended that one of these kits should be used whenever possible as they greatly simplify the bleeding operation and also reduce the risk of expelled air or fluid being drawn back into the system.
14 Connect the outlet tube of the bleeder device to the bleed screw and then open the screw half a turn. Depress the brake pedal to the floor and slowly release it. The one-way valve in the device will prevent expelled air from returning to the system at the completion of each stroke. Repeat this operation until clean hydraulic fluid, free from air bubbles, can be seen coming through the tube. Tighten the bleed screw and remove the tube.
15 Repeat the procedure on the remaining bleed nipples in the order described in paragraph 11. Remember to keep the master cylinder reservoir full.
Bleeding - with pressure bleeding kit
16 These are available from motor accessory shops and are usually operated by air pressure from the spare tyre.
17 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 bubbles are visible in the fluid being expelled.
18 Using this system, the large reserve of fluid provides a safeguard against air being drawn into the master cylinder during the bleeding operations.
19 This method is particularly effective when bleeding “difficult” systems or when bleeding the entire system of routine fluid renewal.
All systems
20 On completion of bleeding, top-up the fluid level to the MAX mark on the reservoir. Check the feel of the brake pedal, which should be firm and free from any “sponginess” which would indicate air still being present in the system.