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2 Hydraulic system - bleeding
Note: Hydraulic fluid is poisonous; wash off
immediately and thoroughly in the case of skin
contact and seek immediate medical advice if
any fluid is swallowed or gets into the eyes.
Certain types of hydraulic fluid are inflammable
and may ignite when allowed into contact with
hot components; when servicing any hydraulic
system it is safest to assume that the fluid is
inflammable and to take precautions against
the risk of fire as though it is petrol that is being
handled. Hydraulic fluid is also an effective
paint stripper and will attack plastics; if any is
spilt, it should be washed off immediately
using large quantities of fresh water. Finally, as
it is hygroscopic (it absorbs moisture from the
air) - old fluid may be contaminated and unfit
for further use. 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
recommended type; never re-use fluid that
has already been bled from the system.
Ensure that sufficient fluid is available before
starting work.
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
proceeding further.
5 Park the vehicle on level ground, switch off
the engine and select first or reverse gear,
then chock the wheels and release the
handbrake.
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 top the master cylinder reservoir 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 further air
entering the system.
8 There are a number of one-man, do-it-
yourself brake bleeding kits currently available
from
motor
accessory
shops.
recommended that one of these kits is used
whenever possible as they greatly simplify the
bleeding operation and also 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 as the procedure
may vary slightly according to the type being
used; generally they are as outlined below in
the relevant sub-section.
10 Whichever method is used, the same
sequence must be followed (paragraphs 11
and 12) to ensure that 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 be
necessary only to bleed that part of the
system (ie. the primary or secondary circuit).
12 If the complete system is to be bled, then
it should be done in the following sequence.
Non ABS models
a) Left-hand rear brake.
b) Right-hand front brake.
c) Right-hand rear brake.
d) Left-hand front brake.
Models equipped with ABS
a) Left-hand front brake.
b) Right-hand front brake.
c) Left-hand rear brake.
of
the
d) Right-hand rear brake.
Bleeding - basic (two-man)
method
13 Collect a clean glass jar, 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. 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.
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 stroke.
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, 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
It
is
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.
Braking system 9•3
19 When no more air bubbles appear, tighten
the bleed screw securely, remove the tube
and spanner and refit the dust cap. Do not
overtighten 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.
Bleeding - 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 more easily
seen 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 and 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 pressure
bleeding kit
24 These kits are usually operated by the
reservoir of pressurised air contained in the
spare tyre, although note that it will probably
be necessary to reduce the pressure to a
lower limit 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 On completion, when firm pedal feel is
restored, wash off any spilt fluid, tighten the
bleed screws securely and refit the dust caps.
29 Check the hydraulic fluid level and top-up
if necessary (Chapter 1).
30 Discard any hydraulic fluid that has been
bled from the system; it will not be fit for re-use.
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
reasonable
repetition
of
the
procedure may be due to worn master
cylinder seals.
9
after
a
bleeding
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