Because it is regularly called upon to work at high temperatures, your brake fluid will lose its effectiveness over time. Most car-makers suggest it is done as part of a major service, which is usually carried out every two years. For example, Volkswagen recommends this, but also says that the first brake fluid change is not needed until the car is three years old. Franchised main dealers usually drain and flush brake fluid using a special machine. But you can do this job yourself, if you have a few bits of specialist equipment and an hour or so on your hands. Firstly though, remember that brake fluid can be corrosive, so you should take care not to get it on your skin. It's also a good idea to wear a pair of disposable gloves while you're doing the job. You can top up your brake fluid without draining your system, and you can buy the stuff from lots of shops which sell car parts.
But brake fluid deteriorates rapidly once it meets the air, because water vapour can cause rust in the system, but it also lowers your brake fluids boiling point. In cold weather, this can lead to ice crystals forming in it, which will eventually turn to water, and will reduce the effectiveness of your brakes. If your car is fitted with ABS, contaminated fluid can damage and even destroy important working parts of such systems.
To change your brake fluid, follow this sequence:
Remove the old, dirty fluid from the master cylinder reservoir. You can do this using a turkey baster,
If you can get access to it, wipe out the reservoir with a lint-free cloth as you slowly empty it.
The system should then be flushed, but this is a job for two people, as you will need someone to loosen the bolts securing the bleeder valves. As these usually go many months without being loosened, they have a habit of becoming jammed. So, they need to be gently manipulated - with the help of a few squirts of lubricating oil to ease them loose if necessary. If you can apply this a couple of days beforehand to give it the chance to penetrate every little crevice, so much the better. Surface rust may also have accumulated on them, so a few gentle taps with a hammer may be called for to disperse this. Loosen the bolts but leave them attached and closed.
Take a piece of clear plastic tubing (for example the type commonly used in an aquarium, which has the extra benefit of being cheap). Push one end of the tube over the brake bleeder bolt at the right rear of the car. Place the other end of the tube in a small, clear bottle with an inch or two of clean brake fluid in it - this will prevent air being sucked back into the brake cylinder or caliper.
Put a small (about 10cm x 2.5cm should do it) piece of wood or similar material under the pedal to prevent the pedal from traveling too far when line pressure is released.
Top up the master cylinder reservoir with fresh fluid, up to the fill line which should be marked on the outside of the pipe, and put the cover back on the reservoir, then immediately replace the cap to prevent fluid from squirting out when you next press the brake pedal. Fluid will squirt out of an open reservoir every time the pedal is released.
As you refill the reservoir, you should take care not to let it get more than half empty.
Enlist a helper to get into the car and press the brake pedal on your cue. When you say so, they should press the pedal with the same amount of force as if they were trying to stop the car from rolling forward. Next, the helper, while keeping their foot on the pedal with the same amount of pressure, gives a cue. You have to then warn them that the pedal is about to sink further down, but they should keep their foot on the pedal, again keeping the pressure constant, while you tighten the bleeder bolt a quarter of a turn. This will release a trickle of old, contaminated fluid, and, as this stops and is replaced by clean fluid, you should close the bleed bolt.