Hydraulic system - bleeding

Braking system / Hydraulic system - bleeding

Warning: Hydraulic fluid is poisonous; wash off
immediately in the case of skin contact, and seek medical advice if any fluid is swallowed or gets into the eyes. Certain types of hydraulic fluid are flammable, and may ignite when allowed into contact with hot components; when servicing any hydraulic system, it is safest to assume that the fluid IS flammable, and to take precautions against the risk of fire. Hydraulic fluid is also an effective paint stripper, and will attack plastics; if any is spilt, it should be washed off immediately, using copious quantities of fresh water. Finally, it is hygroscopic (it absorbs moisture from the air) - old fluid may be contaminated and unfit for further use. Excess moisture con tent lowers the fluid’s boiling point, making it potentially dangerous. When topping-up or renewing the fluid, always use the recommended type, and ensure that it comes from a freshly-opened sealed container.

General
1 The correct operation of any hydraulic system is only possible after removing all air from the components and circuit; this is achieved by bleeding the system.

2 During the bleeding procedure, add only clean, unused hydraulic fluid of the recommended type; never re-use fluid that has already been bled from the system.

3 If there is any possibility of incorrect fluid being already in the system, the brake components and circuit must be flushed completely with uncontaminated, correct fluid, and new seals should be fitted to the various components.

4 If hydraulic fluid has been lost from the system, or air has entered because of a leak, ensure that the fault is cured before continuing further.

5 Park the vehicle on level ground, or alternatively (for better access to the bleed screws) jack up the car and support on axle stands (see “Jacking and vehicle support”) and remove the wheels.

6 Check that all pipes and hoses are secure, unions tight and bleed screws closed. Clean any dirt from around the bleed screws.

7 Unscrew the master cylinder reservoir cap, and if necessary top-up to the “MAX” level line; refit the cap loosely, and remember to maintain the fluid level at least above the “MIN” level line throughout the procedure, or there is a risk of air entering the system.

8 There are a number of one-man, do-ityourself brake bleeding kits currently available from motor accessory shops. It is recommended that one of these kits is used whenever possible, as they greatly simplify the bleeding operation, and reduce the risk of expelled air and fluid being drawn back into the system. If such a kit is not available, the basic (two-man) method must be used, which is described in detail below.

9 If a kit is to be used, prepare the vehicle as described previously and follow the kit manufacturer’s instructions, since the procedure may vary slightly according to the type being used.

10 Whichever method is used, the same sequence must be followed (paragraphs 11 and 12) to ensure the removal of all air from the system.

Bleeding sequence
11 If the system has been only partially disconnected, and suitable precautions were taken to minimise fluid loss, it should only be necessary to bleed that part of the system.

12 If the complete system is to be bled, then it should be done working in the following sequence:
a) Right-hand rear brake.

b) Left-hand rear brake.

c) Right-hand front brake.

d) Left-hand front brake.

Bleeding -
basic (two-man) method
13 Collect together a clean glass jar of reasonable size, a suitable length of plastic or rubber tubing which is a tight fit over the bleed screw, and a ring spanner to fit the screw. The help of an assistant will also be required.

14 Remove the dust cap from the first screw in the sequence (see illustration). Fit the spanner and tube to the screw, place the other end of the tube in the jar, and pour in sufficient fluid to cover the end of the tube.

2.14 Dust cap (arrowed) over the bleed screw on a rear brake wheel cylinder

15 Ensure that the master cylinder reservoir fluid level is maintained at least above the “MIN” level line throughout the procedure.

16 Have the assistant fully depress the brake pedal several times to build up pressure, then maintain it on the final downstroke.

17 While pedal pressure is maintained, unscrew the bleed screw (approximately one turn) and allow the compressed fluid and air to flow into the jar. The assistant should maintain pedal pressure, following it down to the floor if necessary, and should not release it until instructed to do so. When the flow stops, tighten the bleed screw again, have the assistant release the pedal slowly, and recheck the reservoir fluid level.

18 Repeat the steps given in paragraphs 16 and 17 until the fluid emerging from the bleed screw is free from air bubbles. If the master cylinder has been drained and refilled, and air is being bled from the first screw in the sequence, allow approximately five seconds between cycles for the master cylinder passages to refill.

19 When no more air bubbles appear, tighten the bleed screw securely, remove the tube and spanner, and refit the dust cap. Do not over-tighten the bleed screw.

20 Repeat the procedure on the remaining screws in the sequence, until all air is removed from the system and the brake pedal feels firm again.

BIeeding -
using a one-way valve kit 21 As their name implies, these kits consist of a length of tubing with a one-way valve fitted, to prevent expelled air and fluid being drawn back into the system; some kits include a translucent container, which can be positioned so that the air bubbles can be seen more easily flowing from the end of the tube.

22 The kit is connected to the bleed screw, which is then opened. The user returns to the driver’s seat, depresses the brake pedal with a smooth, steady stroke, and slowly releases it; this is repeated until the expelled fluid is clear of air bubbles.

23 Note that these kits simplify work so much that it is easy to forget the master cylinder reservoir fluid level; ensure that this is maintained at least above the “MIN” level line at all times.

Bleeding - using a pressurebleeding kit
24 These kits are usually operated by pressurised air contained in a spare tyre.

However, note that it will probably be necessary to reduce the pressure to a lower level than normal; refer to the instructions supplied with the kit.

25 By connecting a pressurised, fluid-filled container to the master cylinder reservoir, bleeding can be carried out simply by opening each screw in turn (in the specified sequence), and allowing the fluid to flow out until no more air bubbles can be seen in the expelled fluid.

26 This method has the advantage that the large reservoir of fluid provides an additional safeguard against air being drawn into the system during bleeding.

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

All methods
28 When bleeding is complete, and a firm pedal feel is restored, tighten the bleed screws securely and refit their dust caps, then wash off any spilt fluid.

29 Check the hydraulic fluid level in the master cylinder reservoir, and top-up if necessary (see “Weekly Checks”).

30 Discard any hydraulic fluid that has been bled from the system; it will not be fit for reuse.

31 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 required.

Failure to bleed satisfactorily after a repetition of the bleeding procedure may be due to worn master cylinder seals.