Brake Flushing

by Rik Paul

Car Care
by Rik Paul
Motor Trend, APril 1998

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 boiling points. While the two are compatible, DOT 4 has a higher boiling point, so if your vehicle is designed for DOT 4, don't substitute DOT 3. If DOT 3 is recommended for your car, you can use either type. Castrol also markets a brake fluid called LMA, for Low Moisture Absorption. This is formulated to last longer than conventional types because it takes longer to reach its saturation point, and also boasts a higher boiling point.

It's important to note that DOT 5 brake fluid, which is silicon-based, is NOT compatible with conventional fluid and should NEVER be added to a system using DOT 3 or 4. DOT 5, which has a purple color, doesn't absorb moisture, and has a higher boiling point. It's NOT recommended very often for late-model cars because its lack of moisture absorption INcreases the chance of corrosion, and since it can be compressed, it can cause the brake pedal to have a spongy feel. It's also much more expensive.

Certainly never pour any other type of fluid, such as power steering fluid, ATF, or motor oil into the brake system, as those petroleum- based fluids will deteriorate the seals and other rubber components, potentially causing brake failure. In addition, if using DOT 3 or 4, be careful not to spill brake fluid on a painted surface, as it can eat away the paint. If this happens, clean up the spill immediately and wash the area with plenty of water.

Whenever you add the brake fluid, it should come from a fresh, sealed container. This is, again, due to its hygroscopic characteristics. Fluid that has been exposed to air for any length of time can absorb water vapor, reducing its capacity to absorb moisture in the brake system. When buying brake fluid then, always get the smallest container that will do the job. If you're just topping off a master cylinder, for instance, purchase a small bottle. If doing a complete flush, get a large container. Any leftover fluid should be sorted in a tightly sealed bottle and used within a few weeks. It's false economy to use brake fluid that's been sitting around for an extended time.

TIPS for the Do-It-Yourselfer

Brake flushing is much like bleeding air from the system. The most common method is the tried-and-true two-person procedure. It's easier done than described. First remove the lid from the master cylinder (be sure to clean around the lid first, to prevent any dirt from falling into the reservoir). Here, you start at the wheel farthest from the master cylinder, and slip a piece of clear plastic hose over the bleeder valve; it should fit snugly. Place the other end of the hose into a clear containe, such as a glass jar or plastic milk carton, submerged in brake fluid. Have an assistant pump the brake pedal a few times to build up pressure, then on a downward stroke, hold the pedal depressed slightly. While he's holding the pedal down, use a wrench to open the bleeder valve just enough to let a surge of brake fluid escape into the hose and container. Your assistant should then slowly depress the pedal to the floor and hold it there long enough for you to immediately close the bleeder valve again to prevent air from being drawn back into the lines.

The key here is to keep the fluid level in the master cylinder above the minimum mark by repeatedly adding fresh fluid. Once you begin seeing fresh, clear fluid (with no air bubbles) coming out of the bleeder valve in place of the old, dark fluid, you can tighten the valve and move on to the next wheel. Repeat this procedure on all four wheels, working your way toward the master cylinder, which should be your last bleeding point (unless another sequence is recommended in the vehicle's service manual).

In these days of modern technology, there are easier alternatives to this age-old procedure. For instance, you can buy a bleeder hose with a one-way valve, that allows fluid and air out, but doesn't let it back in. This eliminates the need to repeatedly open and close the bleeder valve during each pump of the pedal. Just make sure that the fit of the hose on the valve is tight enough that it doesn't allow any air to seep in through the connection. Russell Performance Products (Dept. MT, 225 Centress Blvd., Daytona Beach, FL 32114; (904) 253-8980) also makes a one-way bleeder valve, called the Speed Valve, with which you can replace your stock valves. This works the same as the one-way hose and eliminates any concern about air leaks at the connections. Either method makes it possible for one person to do all the work, although you still need to check when clear fluid is flowing out of the valve.

Vacuum pumps, such as the popular Mity-Vac models made by Neward Enterprises (Dept. MT, 9251 Archibald Ave., Rancho Cucamonga, CA 91730; (800) 648-9822), can also be used as a one-person flushing procedure by drawing fluid out through each bleeder valve, without the need for someone to pump the brake pedal. The company also markets the Mityfill Automatic Fluid Feeder, which can be attached to the master cylinder to provide a constant supply of fresh brake fluid as you're removing the old fluid, eliminating the need to be constantly check the fluid level.

Perhaps the most sophisticated tool for both bleeding and flushing brake systems is the Phoenix Injector, produced by Phoenix Systems. This has the capability of either injecting fluid into the system or pumping it out, to allow a variety of one-person techniques to be used. For instance, in split-system designs (most vehicles produced in the last couple of decades), you can pump fresh fluid into one bleeder valve, while the old fluid is simultaneously extracted from the opposing valve. Or the system can be pressure flushed (a professional technique normally requiring expensive hardware) by pumping fluid through the system from each master cylinder reservoir port, while again collecting the old fluid at each bleeder valve. Or it can be used to vacuum flush a system, acting as a conventional vacuum pump. For bleeding, the Injector also allows reverse fluid injection, which removes air bubbles by forcing them upward and out of the master cylinder, requiring a minimal loss of fluid. The Phoenix Injector is a professional-caliper instrument, and understandibly isn't cheap; depending on model, it runs $70 to $305.

Regardless of which flushing method you choose, when you're done, be sure to top off the level in the reservoir and install the lid securely. Assuming all mechanical components are in order, your brake pedal should have a strong, firm, confident feel, and the system should be working at maximum efficiency, ensuring you'll get the most from your brakes.

TIP of the Month: The Masters of Our Own Cylinders Even if there are no leaks in your car's brake system, the fluid level in the brake master cylinder will still gradually go down. This is important to keep track of, because it is a good indication that it's time to check the vehicle's brake pads and/or shoes for wear.

As the friction lining on pads and shoes wears down, they are pushed out farther to maintain good braking contact with the rotors or drums. This, in turn, requires that more fluid be drawn from the master cylinder. Eventually, this will be evident in the master cylinder reservoir as the fluid level drops to near the minimum mark.

Be sure to check your brakes at this point, and don't forget to top off your fluid.

Unca Joel's warning:

If you add fluid BEFORE you replace the pads or shoes, you may regret it later (after you've forgotten that you added that fluid!) when you do change the pads/shoes. the new pads and shoes are thicker, and will push the fluid back up into the master cylinder reservoir ... and all that fluid you added may spill out!!!

So if you're gonna add fluid, make yourself a note somewhere to watch out for spillage when you replace the pads/shoes. :)

For Vanagons You have FIVE bleeder valves. The fifth one is on your hydraulic clutch slave cylinder (on the transmission, between the rear wheels). This fluid also needs to be bleed/flushed periodically (whenever you do the wheel cylinder bleeder valves). I think it's easier to reach from up top, through the engine compartment. And yes, you have to pump the CLUTCH pedal (instead of the Brake Pedal). :)

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