All the car braking systems of today are hydraulic, which control braking action through fluid pressure. The periodic changing of brake fluid, is a largely ignored maintenance chore, due to the little or no mention it gets in automobile manuals.
Why is it Important?
The primary reason for a periodic replacement is the chemical nature of the glycol-based braking fluids. These chemicals are hygroscopic, that is, they have a tendency to absorb moisture. With moisture absorption, boiling point of these fluids goes down and the efficiency of braking also gets compromised.
When the boiling point drops, the brakes don't work properly in high temperature conditions or high braking situations. Fresh fluid is clear, while the one which has absorbed moisture, is denser and less clear. This deterioration of quality can be especially detrimental to anti-lock braking systems.
There is no way that one can totally stop the entry of moisture into the braking system, as the whole system is exposed to the atmosphere, through venting. During each opening of the master cylinder, a little moisture is absorbed. However, the process of absorption is slow and if you schedule a regular fluid flush, then this problem of brake fading, due to moisture absorption, can be kept at bay. Flushing guarantees that your brakes have a shorter response time and they work perfectly when you need them.
Choice of Fluid
It is important that you replace the old fluid, with right type. They are classified into three main types, which are DOT 3 (lower boiling point temperature threshold is 400 Degree Fahrenheit), DOT 4 (lower boiling point temperature threshold is 450 Degree Fahrenheit), and DOT 5.1 (lower boiling point temperature threshold is 500 Degree Fahrenheit). The DOT 5.1 fluids are mostly used in racing cars. Check the kind of fluid used before and replace it with the same kind. Make sure that the new fluid can is sealed until you are ready to use it.
The procedure is similar to the technique of bleeding brakes. The things you'll need for the procedure are a plastic tubing, box wrench, turkey baster, a clear plastic bottle, and a new fluid. You will also need another person to help you out. Wear hand gloves and safety glasses.
To gain access to the brake mechanism underneath, use a jack. Use two jacks, as they can effectively support the elevated vehicle. You must carry out the flushing exercise for one wheel brake at a time. First part of flushing is opening the master cylinder and draining the fluid using a turkey baster. Then, clean the insides thoroughly and make sure that no contaminants are left inside.
Pour the new fluid inside the master cylinder and top it up. Ask somebody to sit in the driver's seat and wait for your instructions. Locate the bleeder valve in the braking system. Attach a clear plastic tubing to the bleeder valve and dip its other end into a clear bottle, that is partially filled with the braking fluid.
Ask your helper to press down slowly on the brake pedal, without flooring it. Once he has pressed it down, open the bleeder valve by a quarter turn and drain the fluid. Then, close the valve and ask your helper to release the brake pedal. Repeat the process until clear fluid starts streaming out of the master cylinder. At periodic intervals, replenish the master cylinder, with the fluid. Repeat the procedure on rest of the wheel braking systems.
Nowadays, specially designed flushing tools and machines are available, that can carry out the job even easily.