Bleeding the brakes and clutch
Trevor Cuthbert explains the methods and equipment that can make brake and clutch bleeding an easy one-person task
The methods and tools that can make brake and clutch bleeding an easy one-person job
The braking system on a Land Rover uses a master cylinder operated by the brake pedal to force fluid through the brake pipes to the brake callipers (on disc brakes) or the brake cylinders (on drum brakes). The hydraulic action and force exerted by the brake fluid on the pistons in the calliper squeezes the brake pads on either side of the brake disc to operate the brakes at each of the wheels. Likewise, with drum brakes, the fluid hydraulically acts on the brake cylinder, which pushes the brake shoes outwards against the brake drum. Drum brakes are found at the front and rear of Series Land Rovers, while later coil-sprung Land Rovers have disc brakes at the front and can have disc or drum brakes at the rear, depending on age and model.
The clutch operates by a similar hydraulic system, where a pedal-operated master cylinder pushes clutch fluid through to the slave cylinder, to ultimately push the clutch release lever against the pressure plate to engage or release the clutch.
The braking and clutch systems use the same type of hydraulic fluid – DOT 4 brake fluid – and rely on only fluid being in the system. If there is any air in the system, it will not operate correctly because when the pedal is pressed, the air in the system will compress before the fluid does, resulting in a spongy brake pedal – this is almost always due to the presence of some air in the hydraulic system.
Need to bleed When the hydraulic system is opened during maintenance or repair, air becomes introduced. This air must be expelled and replaced with brake fluid by bleeding the system. Examples of repairs that will introduce air include, renewing rusty or fractured brake pipes, replacing flexible brake hoses, replacing brake calipers (or wheel cylinders in drum brakes), replacing a master cylinder or slave cylinder, and replacing all of the fluid with new as part of a service schedule.
Conventional brake and clutch bleeding Bleeding the brakes or clutch by the traditional method, requires two people. One to press the brake or clutch pedal at the appropriate time, while the other person is at the caliper, wheel cylinder or clutch slave cylinder with a jar, a length of clear tube and an 11 mm spanner.
Before bleeding, the fluid reservoir must be filled, and kept topped up throughout the process – otherwise air will be reintroduced. The pedal is pressed to the floor and held there. At this point the other person uses the spanner to open the bleed nipple, so that air and fluid are expelled through the tube and into the jar (the clear tube passes below the level of fluid in the jar to prevent air getting in at the end). The bleed nipple is closed again, before the pedal is released. This process is repeated until only clear brake fluid is expelled through the tube, which indicates all the air has been pumped out and replaced with fluid.
This traditional method is very effective and has been used since the dawn of hydraulic brakes and clutches – and some would argue that it is the best method. However, it requires two people to operate, and can be time-consuming.
ABS brakes The traditional brake bleeding method is not suitable for ABS braking systems. These systems are much more complicated and are usually bled by a ‘power bleeding’ method using software built into the vehicle ECU via a diagnostic unit. This allows for one-person bleeding. Alternatively, ABS braking systems in Defender and Discovery 2 can be bled by the two methods described here, if a diagnostic unit is not available.