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Brakes won't work properly if air gets into their hydraulic system.
This can happen when the level of the master cylinder's reservoir of brake fluid drops too low or when part of the hydraulic system, such as the flexible brake hoses or a wheel cylinder, was removed for replacement or repairs.
Here are some tell-tale signs that there is air in the system.
l The brakes feel "spongy";
l You have to press the brake pedal very far down before the brakes start to grip;
l The brakes suddenly seem to lose efficiency.
Air in the brake lines causes so many problems because air is compressible whereas brake fluid, being a liquid, is not.
With hydraulic brakes the slightest movement of the brake pedal should be transferred to the pistons inside the wheel cylinders by an incompressible fluid. But when air is mixed with the fluid, the first part of the pedal's movement is used to compress the air enough to overcome the resistance of the pull-back springs holding the brake shoes away from the drums.
Air in the brake lines is not the only thing posing this danger. If the brake fluid gets hot enough to start boiling, you get vapour in the system, which will produce the same symptoms as air.
If this continues for too long, the boiling point will drop far enough for the "wet" brake fluid to start boiling.
So, if you want to get rid of air in the system, or forestall problems caused by wet brake fluid, you must flush clean fluid through the brake lines regularly.
This is called "bleeding the brakes" and is quite easy to do.
First locate the so-called bleed nipples. On drum brakes these are accessible from the back plate. They screw into the wheel cylinders, close to where the steel pipe carrying the brake fluid enters.
On disc brakes the nipple is on the caliper.
Move to the wheel furthest from the master cylinder. Place a ring spanner of the right size on the hexagonal section of the nipple, then push one end of a flexible, transparent plastic or nylon hose, about one metre long, onto the nipple, ensuring that it's a tight fit.
The other end of the hose must be deep in a glass jar containing enough brake fluid to cover the end of the hose. Get a helper for the tasks of pumping the brake pedal and keeping the master cylinder reservoir topped up with clean brake fluid at all times.
With everybody ready and a litre of fresh brake fluid at hand, unscrew the bleed nipple by one turn. Be careful: an ill-fitting spanner or one that slips can round off the nipple and cause further problems.
If the nipple is stubborn, a sharp knock on a well-placed spanner is better than excessive shoving. Once the nipple has been loosened, instruct your helper to start pumping the brake pedal with full strokes, allowing it to return slowly. If there was air in the system, continue until the fluid flows from the drain tube in a solid stream, free of air bubbles.
If the purpose is to replace the brake fluid with fresh fluid, continue until the colour of the emerging fluid changes from the darker colour of old fluid to the lighter colour of fresh fluid.
Never allow dirt, oil or petrol to get into the fluid reservoir. Keep the end of the drain hose below the fluid level in the jar to prevent air from being sucked in on the return strokes of the pedal.
Once the job is done at the first wheel, remove the drain tube after tightening the nipple firmly but not too tightly.
Repeat the procedure on the other wheels, finishing with the wheel closest to the master cylinder.
T ry out the brakes at low speed. If not satisfied, bleed again.