Flushing Brake Systems

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Flushing Brake Systems

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Harley Davidson Forums >> Do It Yourself Help Forum >> Chassis, Suspension, Brakes, and Front End/Controls >> Flushing Brake Systems

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Flushing Brake Systems - 2/26/2006 1:26:14 PM

HDF Tech

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Are you getting all of your stopping power?? The importance of brake flushing

Brake fade is somewhat akin to brain fade. Reaction time is slower, usually snappy performance is replaced by a dull, mushy feel, and in critical situations the slow response time can be disasterous. While there are several mechanical reasons for this type of condition, one of the most overlooked causes is contaminated brake fluid. Most car owners are well aware of the importance of maintaining proper fluid levels in the vehicle, and reliably check to make sure that the brake fluid is mainained between the minimum and maximum marks on the reservoir. After all, if there's too little fluid, it's possible for air to be sucked into the lines, which can result in a spongy pedal feel and inefficient braking performance.

The importance of flushing the brake system ... taking the old fluid and replacing it with new ... is often not so well recognized. In fact, a few years ago, Motor Trend Senior Road Editor Mac DeMere needed to get his vehicle's brake system flushed and call a local gas station for a price. The person he talked to ... even with never having seen Mac's car ... told him that procedure wasn't necessary. When Mac pressed for a price anyway, the person again fervently stated that his vehicle's brake fluid didn't need to be changed. Finally, Mac simply called elsewhere. One of the problems is that flushing the brake system if often not specifically listed in many vehicle's routine maintenance schedule, which could cause an unknowledgeable person to think that the procedure wasn't necessary. In fact, manufacturers usually specify a general inspection of the brake system and leave it up to the mechanic to know when to flush the fluid.

Why is flushing important? Conventional glycol-based brake fluid is hygroscopic, which means that it absorbs water. This is important, in order to keep condensation in the brake system from causing corrosion. However, eventually, the fluid will absorb all the moisture it can hold, reaching its point of saturation. Several things can happen at this point. Unabsorbed moisture can begin to collect in the system, causing corrosion in critical areas; the water can cause seals to swell and deteriorate, further contaminating the fluid; and the boiling point of of the fluid drops beyond recommended levels. This means that under high-heat braking conditions, such as during hard braking or repeated brake application while descending a mountain, the fluid will start boiling sooner, which will reduce braking performance. The pedal can begin to feel spongy, and as braking efficiency drops, it takes longer to stop the vehicle.

How do you know when the system should be flushed? Most maintenance schedules that do specify it recommend changing the fluid every two years or 30,000 miles. If you live in an unusually humid climate, it's better to plan on doing it every year. However, your eyes can tell you when the time has come. Fresh brake fluid is transparent and has a slight amber-colored look. As the fluid absorbs moisture, it takes on a darker, cloudy appearnace, which tells you it needs changing. This can also be verified empirically. For instance, Phoenix Systems (Dept. MT, 3555 E. 42nd Stravenue, Tucson, AZ 85713; (888) 749-7977), a company that specializes in professional brake system tools, markets a Brake Fluid Tester ($370) that reads the boiling point of a sample of fluid. Fresh brake fluid has a boiling point of about 400 degrees F or higher. As it absorbs moisture, however, the boiling point will gradually drop. Phoenix Systems recommends that if its tester shows a fluid's boiling point to be below 330 degrees, the system should be flushed.

When adding new brake fluid, be sure it's the appropriate type for your vehicle; it will be listed in the owner's manual or marked on the reservoir cover. Conventional glycol-based formulas are designated as DOT 3 and DOT 4, which differ mainly by their boi