Friday, April 30, 2010

Bleeding the brakes

If the brake pedal feels soft and spongy, then there is air within the system. Unlike brake fluid, air can be compressed, and the soft feel of the pedal is due to the fact that pushing it is compressing air rather than pushing non-compressible fluid to operate the brakes. In this case, the brakes have to be bled. The brakes also have to be hied after any part Of the hydraulic system
has been temporarily disconnected.Bleeding the brakes entails pumping fluid through the pipes until the fluid which contains air bubbles is removed from the system via one of the bleed valve nipples. The nipple is turned to allow fluid to escape as the pedal is pumped, then tightened before the pedal returns. To prevent air from re-entering the system via the bleed nipple, a short length of transparent plastic pipe is attached to it and the other end is immersed in a container of clean brake fluid.
If you have to bleed the whole system, begin with the wheel furthest from the master cylinder and work forwards (single circuit) or, in the case of dual circuit systems (in which you'll have a tandem master cylinder and front and rear brakes with individual circuits) bleed nearside front, offside front, nearside rear and finally offside rear.
To bleed a brake you will require an assistant to push the brake pedal for you. Attach the pipe to the brake nipple and immerse the other end of this in a small container of clean brake fluid. Open the nipple by turning it and call for the brake pedal to be depressed and held down. When your assistant has done this, tighten the nipple, and then ask your assistant to release the pedal. Repeat the exercise until clear fluid with no air bubbles can be seen coming from the nipple. Ensure throughout this operation that the level of the fluid in the master cylinder is correctly maintained.
Sometimes air can become lodged somewhere in the system and refuse to be bled in the normal manner. If this happens the pedal will feel spongy. The air can usually be dislodged by going through the motions of bleeding the brakes, but ask your assistant to let his or her foot slide off the brake pedal so that it returns sharply. A good brake pedal is nice and solid, and has not too much void travel, so that if one circuit fails (tandem master cylinder) then the other will still operate. If the pedal travel is too great, check firstly that the rear drum brakes arc correctly adjusted, then adjust if necessary the master cylinder push rod. if the rear brakes are not correctly adjusted –and even one shoe out of
adjustment will cause this – then the pedal will travel some distance as the shoe(s) concerned is moved by the pedal. You can usually hear a sound as the shoe moves and contacts the brake drum when this happens.
You don't necessarily have to jack up the rear of the car to adjust the handbrake; it is possible to reach the adjuster holes by lying on the ground at the rear of the car (watch out for the hot exhaust!). (Remember the advice given in Chapter Three about using a nut and bolt to remind you of which way to turn each adjuster.) It is very easy to become confused and to wind an adjuster fully in. and to believe that it is fully out! in addition to the extra brake pedal travel which will highlight the error, the handbrake lever will also possess far too much travel.
Brake bleeding is one of those activities which goes without a hitch nine times nut of ten – on the tenth occasion, it can be frustrating when you are unable to obtain a good solid feel to the pedal. The tenth occasion usually follows a full restoration, when you are racing against time to get the car ready for the scheduled MOT test. Don't carry on feeding good brake fluid through the system; it's expensive and should never be re-used. Try using a brake hose clamp (or two on front then rear, in the case of dual-circuit systems) on each flexible hose in turn, and feel whether this makes any difference to the pedal. The chances are that when one particular hose is clamped, the pedal suddenly behaves perfectly, showing you which wheel cylinder or calliper still has air inside.
If, after all your attempts to bleed the system, the pedal is soft, then get a second opinion from an experienced mechanic. The chances are that the master cylinder needs attention in the form of new seals, although dirt in the system. persistent air bubbles, One-man brake bleeding kits, which are fitted with a small non-return valve to prevent air from being sucked back into the system, are available. The author has tried various of these and found that, while some work
satisfactorily, some of the cheaper kits give problems with the non-return valve. As ever, you are tdvised to buy the very best tools which you can afford.

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