Brake hydraulic system (conventional braking system) - bleeding

General
1 If any of the hydraulic components in the
braking system have been removed or
disconnected, or if the fluid level in the
reservoir has been allowed to fall appreciably,
it is inevitable that air will have been
introduced into the system. The removal of all
this air from the hydraulic system is essential
if the brakes are to function correctly, and the
process of removing it is known as bleeding.

2 Where an operation has only affected one
circuit (front or rear) of the hydraulic system,
then it will only be necessary to bleed the
relevant circuit. If the master cylinder has
been disconnected and reconnected, or the
fluid level has been allowed to fall
appreciably, then the complete system must
be bled.

3 One of three methods can be used to bleed
the system.

Bleeding
Two-man method
4 Gather together a clean jar and a length of
rubber or plastic bleed tubing which will fit the
bleed screws tightly. The help of an assistant
will be required.

5 Remove the dust cap where fitted, and
clean around the bleed screw on the relevant
caliper or wheel cylinder, then attach the
bleed tube to the screw (see illustration). If
the complete system is being bled, start at the
front left-hand caliper.

2.5 Removing the dust cap from a wheel cylinder bleed screw

6 Check that the fluid reservoir is topped up
and then destroy the vacuum in the brake
servo by giving several applications of the
brake pedal.

7 Immerse the open end of the bleed tube in
the jar which should contain two or three
inches of hydraulic fluid. The jar should be
positioned about 300 mm (12.0 in) above the
bleedscrew 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, after the bleed screw is
retightened, quickly remove his foot to allow
the pedal to return unimpeded. Repeat the
procedure.

9 Observe the submerged end of the tube in
the jar. When air bubbles cease to appear,
tighten the bleed screw when the pedal is
being held fully down by your assistant.

10 Top-up the fluid reservoir. It must be kept
topped up throughout the bleeding
operations. If the connecting holes to 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 Assuming that the complete system is
being bled, the procedure described in the
preceding paragraphs should be repeated on
the front right-hand caliper followed by the
rear right-hand and left-hand wheel cylinders.

12 On completion, remove the bleed tube,
and discard the fluid which has been bled
from the system unless it is required for bleed
jar purposes. Never re-use old fluid.

13 On completion of bleeding, top-up the
fluid level in the reservoir. Check the action of
the brake pedal, which should be firm and free
from any “sponginess” which would indicate
that air is still present in the system.

With one-way valve
14 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.

15 Proceed as described in paragraphs 5
and 6.

16 Open the bleed screw half a turn then
depress the brake pedal to the floor and
slowly release it. The one-way valve in the
bleeder device will prevent expelled air from
returning to the system at the completion of
each stroke. Repeat this operation until clear
hydraulic fluid, free from air bubbles, can be
seen coming through the tube. Tighten the
bleed screw.

17 Proceed as shown in paragraphs 11 to 13.

With pressure bleeding kit
18 These too are available from motor
accessory shops and are usually operated by
air pressure from the spare tyre.

19 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.

20 Using this system, the large reserve of
fluid provides a safeguard against air being
drawn into the master cylinder during the
bleeding operations.

21 This method is particularly effective when
bleeding “difficult” systems or when bleeding
the entire system at time of routine fluid
renewal.

22 Begin bleeding with reference to
paragraphs 5 and 6 and proceed as described
in paragraphs 11 to 13.

Cleaning the exterior
WARNING:
If you use a car wash with a waxing
cycle, make sure that you remove
the wax from the windscreen.
CAUTIONS:
Prior to using a car wash facility
check the suitability of it for your
vehi ...