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What’s the best way to bleed brakes?

Bleeding brakes

Martynn Randall is technical editor at Haynes and has been with us for 27 years. He's written more than 60 Haynes publications and has owned more than 85 cars and 60 motorbikes... so far!

 

Bleeding brakes to rid the hydraulic system of air is essential whenever the fluid is changed. Although the traditional method of using the brake or clutch pedal to bleed the hydraulic circuit is still used, there are certain aspects that could cause problems for the home mechanic.

Specifically, forcing the master cylinder piston to operate to its full extent of travel, could cause the rubber piston seal to squeeze through a rarely used (possibly corroded) section of the master cylinder bore. This could damage the seal, or accelerate its wear.

Another potential issue is that on the brake or clutch pedal upstroke, it’s possible for air to enter the hydraulic system where the bleed screw/nipple screws into the caliper or slave cylinder. This should only happen when the coordination between the pedal operation and bleed screw/nipple closing is poor, but it’s mighty frustrating when it does.

Gunson Eezibleed

A much safer and more successful method is to pressurise the hydraulic circuit and the fluid reservoir with low-pressure air, using kit such as the Gunson Eezibleed shown here. Others, such as the Laser Manual Brake Bleeder, are available.

The bleeding kit includes a pressurised fluid container which automatically tops up the master cylinder fluid reservoir as fluid is bled. So there’s no risk of air ingress, no risk of the reservoir running dry, no need to follow a set bleed screw/nipple opening sequence, and it’s a one-man operation.

The cherry on the top is that this method is much better at removing any stubborn air pockets from the hydraulic circuit than the traditional pedal-pumping method.

Bleeding brakes tyre pressure Gunson

The Gunson's pressurised air is provided by connecting the bleeding kit to one of the tyres. Ideally, use the spare tyre, but any will do – note that the pressure in the tyre may have to be reduced to about 1.4 bar (20 psi).

The kit includes various adapters which fit on to the master cylinder fluid reservoir filler neck. The fluid container supplied is then filled with new brake fluid, and the pressure hose connected to the tyre. Laser's brake bleeder features a hand pump, which does away with the need for a tyre.

With the system pressurised (a good seal with the screw cap on the brake reservoir is vital), open a bleed screw/nipple and wait until clean, bubble-free fluid emerges. Then close the screw/nipple, and move onto the next one. Job done!