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How To Flush Your Braking System

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On: Fri, Sep 23, 2011 at 2:00PM | By: Peter C Sessler

How To Flush Your Braking System

Your vehicle’s brake fluid should be changed on a regular basis, at least every two years.

Brake fluid never goes bad, it is hydroscopic—meaning it has a tendency to absorb moisture—something it’s supposed to do anyway. But it doesn’t take much moisture to make the fluid less effective. The heat generated by normal and especially hard braking makes the moisture in the fluid boil and that causes brake fade. And, what’s worse, brake fluid that is contaminated with moisture and dirt (if moisture gets in, dirt will get in the system, too) can have a very detrimental effect on braking systems that use anti-lock brakes. The contaminants can ruin the delicate innards of these systems, which are extremely expensive to repair.

You can’t really stop moisture from entering the brake system because the system is vented. The rubber brake hoses used are also permeable. So every time you open the master cylinder to check or add fluid, moisture is absorbed. Since the contamination process is a very, very slow one, it isn’t something to worry about, but flushing your brake system is a good thing to add to your automotive maintenance list.

Flushing the System
You’ll need some hose to fit on the brake bleeder valves (preferably clear), a container to catch the fluid, a brake bleeder wrench, and a can of spray brake cleaner. You’ll also need a container or two of fresh brake fluid. Brake fluid is classified as DOT 3 (minimum boiling point of 400-degrees), DOT 4 (450-degrees), and DOT 5 or 5.1 (500-degrees). Practically every car uses DOT 3 or 4—don’t use the DOT 5 or 5.1 silicone brake fluids. They are not compatible with regular fluids and have several disadvantages that make them unsuitable for street use (although some racecars use them).

Start by cleaning the master cylinder by spraying it with the brake cleaner. This eliminates the possibility of dirt entering the system. Open the master cylinder and draw out the old fluid—use a turkey baster. Then fill the cylinder with fresh fluid.

Next, jack the car up so you can gain access to each brake. Carefully loosen the bleeder valve, slightly. Always use a tubing wrench here—regular wrenches won’t work. If you can’t loosen the valve, try some penetrating oil or even heating it up. A propane torch works here (but a lighter will do in a pinch). Hopefully, that’ll be enough. If you manage to snap the bleeder screw off, you’re in a bit of trouble.

Most cars today have a diagonally-split system and you should work with each diagonal (for example, the left front wheel and the right rear wheel may be actuated from one of the reservoirs in the master cylinder, and the right front and left rear from the other. Some cars have a front and rear split). If you don’t know, just start with one wheel and then go on to another.

You’ll need an assistant to do this correctly, although there are one-person brake bleeding kits available. The way I do it is to have the assistant press on the brake pedal upon my call. To avoid bottoming-out the pedal completely, place a small piece of wood behind the brake pedal. When the assistant presses down on the pedal as far as it can go, have him/her call out "Down." As the pedal is pushed down, open the brake bleeder valve to let the old fluid out into the container.

When the assistant calls out that the pedal is down, immediately close the valve. Repeat the process until the fluid that comes is clear (you’ll have to refill the master cylinder several times). Then go to the next wheel and repeat the process. Eventually, clear fluid will come out of every bleeder valve.

Cars with anti-lock brakes may require a slightly different procedure on the rear brakes. Because the system is pressurized, have your assistant turn the car on, press on the brake pedal, turn off the ignition, and then just rest his/her foot on the pedal. Open the bleeder valve and let the fluid stream out for about ten seconds or so. Close the valve and add some more fluid at the master cylinder.

On some systems, the assistant may have to keep pushing down on the pedal to let the old fluid out.

After you’re done, have your assistant step on the brakes two to three dozen times to exhaust the high-pressure reservoir on ABS systems. This will make the brake fluid level rise in the master cylinder. When it stops rising, you’re done. To get rid of the brake warning light that’ll be on at this point, step on the brakes hard.

You don’t need to do this every time your vehicle has a brake job, but every third time is a good idea (if you keep the vehicle that long).


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