Bleed­ing the brakes and clutch

Trevor Cuth­bert ex­plains the meth­ods and equip­ment that can make brake and clutch bleed­ing an easy one-per­son task

Land Rover Monthly - - Contents -

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.

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