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Power-Bleeding BrakesAir today, gone tomorrow
Connected to the brake pedal is a rod that pushes a piston inside the master cylinder. When the brake pedal is pushed down that piston pushes brake fluid through the brake lines. Since this fluid has nowhere to go it pushes pistons inside the brake calipers or wheel cylinders. These pistons push brake pads or shoes against the rotating discs or drums and presto-the vehicle stops. Brake fluid, by virtue of being a non-compressible hydraulic fluid, transfers the pressure between the brake master cylinder and the pistons inside the calipers or wheel cylinders.
Every time work is performed on a brake system it's a good idea to bleed the system of any air that may have entered the system. The reason is simple. Air can be compressed, where as brake fluid cannot. Trapped air in the brake hydraulic system can cause a myriad of problems, from a low or spongy brake pedal feel to outright system failure.
Bleeding the brakes removes any air that may have snuck in while working on the brakes and returns the system to its non-compressible state. While there are many methods to bleeding brakes by far the most efficient and easiest is to pressurize the entire system from the master cylinder reservoir and then bleed normally. This method not only purges the entire system of any trapped air, but also flushes out any possibly contaminated brake fluid and can be performed by one person.
Just like every other part of the brake system, brake fluid wears out and needs to be replaced on a regular basis. A more accurate description is that the fluid becomes contaminated with moisture. Over time moisture enters the brake system because brake fluid is highly hygroscopic, which by definition means it readily absorbs moisture from the atmosphere.
Moisture, or water, in the brake fluid creates issues on two levels: both of which can lead to serious problems. Along with being non-compressible brake fluid is manufactured to resist turning from a liquid into a gas - or boiling. If the brake fluid absorbs too much water, its boiling point can drop to a dangerously low level. Water turns from liquid to gas at 212 degrees, a temperature that brakes can rapidly reach. If the brake fluid has reached the point where its boiling point is compromised, it will vaporize when brakes get hot, and can cause brake failure. Ironically the boiling of this moisture also creates vapor, or air, within the brake system, and may be where the air in the lines came from in the first place.
Another problem with contaminated fluid is that of corrosion. Water in the brake fluid makes it corrosive to every part of the brake system. Regular flushing of brake fluid can prevent damage and expensive repairs.
On the DOT
When it comes to bleeding brakes and replacing brake fluid always use new brake fluid with the correct DOT rating. DOT is an acronym for the Department of Transportation.
Brake fluid is classified as DOT3, 4, 5 and so on. What the DOT ratings specify is the minimum boiling point of the brake fluid. Be wary of using some DOT 5 silicon-based brake fluids. These fluids get around the problem of water absorption by being not at all hygroscopic. While this solves one problem it creates another. Any water absorbed into the brake system settles down at the lowest point. It could not possibly settle in a worse place, as the lowest point is usually the calipers, which sit directly on top of the hottest part of the brakes.
While this may all seem confusing, the only strategy is to use only the DOT fluid rating recommended by the brake system manufacturer. Never use old brake fluid from a previously opened container. If the container has sat open, even with the cap on, it has probably already absorbed too much moisture to be used safely. Another important thing to remember about brake fluid is that is will destroy painted finishes. When working with brake fluid always keep plenty of cool, clean water nearby to immediately flush any spills. Always dispose of used brake fluid properly.
Brake by Numbers
Oddly enough most motorists don't consider flushing out the old brake fluid when replacing pads or calipers, or even when bleeding brakes. By using a power bleeder the entire system can be bled and flushed at the same time. The good news is these systems are now available at a price affordable to the do-it-yourselfer.
When bleeding brakes the general rule is to start by bleeding the caliper furthest away from the master cylinder then work your way towards the one closest to the master cylinder. The power bleeder makes this task easy by pressurizing the entire system, and avoids the problem of running the reservoir dry and introducing more air into the system as it holds up to two quarts of brake fluid.
The electronic module controls the splitter, so that it engages at the proper speed, and turns off automatically (at about 20 mph), in case you forget. The module also locks and unlocks the factory torque converter in an automatic, and also locks out the overdrive when in four-wheel-drive. Overall, the gear splitter integrates seamlessly with the factory driveline, and is easy to operate when it comes time to make your move around that semi that gets in your way.
Once the system is pressurized, simply hook up a clear hose and catch container to a bleed screw. Open the system at that point by turning the bleed screw. Allow line to bleed until clear, bubble free brake fluid is seen running though the hose. Repeat this procedure for each wheel, and then disconnect the power bleeder. ABS equipped systems may require an additional step.
When dealing with brakes always follow the recommendations of the owner's or service manual. Follow along with the step-by-steps for a look at how to bleed brakes like a pro using a power bleeder.
Motive Products (http://www.motiveproducts.com)