MotoMojo: Bleeding Brakes

Apr 15, 2014 View Comments by

MotoMojo: Bleeding BrakesThe polyethylene glycol-based brake fluid that comes in our motorcycles is hygroscopic, which means that it attracts moisture from the air. Over time, the fluid absorbs more water, which lowers its boiling point. This can result in a total loss of brakes, due to fluid vaporization, and is likely to happen during hard braking when you need them most. For each percent of water absorbed, the fluid’s boiling point drops about 50 degrees Fahrenheit. Absorbed water also causes the corrosion of expensive and vital internal brake components. If you live in a humid climate, the problem is worse.

In order to keep the system working at peak efficiency, your motorcycle must have fresh brake fluid that’s free of dirt and moisture, as well as no air in the lines. Therefore, it’s recommended that brake fluid be replaced and the system flushed about every two years. A good time to do this is in the spring, before the summer riding season starts.
Brake bleeding on most motorcycles with conventional hydraulic brakes, which are not linked or do not have ABS, can be done in a straightforward manner. The procedure doesn’t require a lot of fancy tools or skills.

Combined (linked) brakes and ABS brake bleeding procedures are outlined in the service manual. For example, on a linked system, there may be an additional bleeder screw on the front caliper, which is bled along with the rear brake. On Harley-Davidsons with ABS, a tool called “digital technician” (available at dealers) is needed to eliminate air bubbles from the system. If you just bleed the brakes to replace fluid, the “digital technician” may not be required, as long as you don’t introduce air into the system. It’s generally the same with other brands with ABS. However, before you begin, consult the owner’s and service manual for your make and model. With any model, if you can’t get the brakes to feel as they were before bleeding, take the machine to a dealer for service before riding. BMW Servo assist systems should be bled by a dealer.

Types of Fluid
Use the type of brake fluid recommended by your motorcycle’s manufacturer. Glycol-based fluids are designated as DOT 3, DOT 4, and DOT 5.1, while silicone-based synthetic fluid is designated DOT 5. DOT 5.1 is actually a non-silicone version of DOT 5, defined by FMVSS 116 as containing less than 70 percent silicone. DOT 5 silicone brake fluid, which should not be used with ABS and must not be mixed with glycol-based fluids, is also known to provide a spongier brake feel.

Minimum dry/wet boiling points in degrees Fahrenheit for these specifications are as follows: DOT 3, 401/284; DOT 4, 446/311; DOT 5, and 5.1 500/356. (Wet boiling point defined as 3.7 percent water by volume.)

You can use a clear plastic tube run into a clear container such as a plastic water bottle to bleed the fluid into. The following instructions are for this basic method. Motion Pro and Russell sell bleeder kits with one-way check valves that prevent air from back-flowing into the system. Optionally, a power bleeder such as Mityvac, which pumps fluid from the brake bleeder, can make bleeding easier and faster. When you fill the master cylinder and open the bleeder, a partial vacuum draws fluid and air out of the system into a container.

Caution: Wear eye protection and nitrile gloves. Brake fluid can damage eyes or paint; rinse spilled fluid profusely with water. Never operate a motorcycle if the brakes feel spongy or have more lever travel than before bleeding.

Start with the rear brake, which is simpler and always has just a single caliper. Position rags around the reservoir to catch any spillage and protect paintwork (gas tank, fairings, etc.) from spills. Remove the reservoir lid and suction the old fluid out. I use a new turkey baster, which I bought at a dollar store. Refill with fresh fluid.

With the hose connected to the bleeder, pump the brake lever three times, then hold it in the applied position and slowly open the brake bleeder. Operate the brake lever through several strokes and watch the fluid color in the hose. Old, dark fluid should begin to be replaced by clear fresh fluid. Close the bleeder. Refill the reservoir as needed; never let the master cylinder run dry. Repeat until no air bubbles are seen. Keep the lever pressed as you close the bleeder; this prevents a backflow of air bubbles. You may have to repeat this procedure a number of times until you achieve a firm, hard lever feel. Replace the dust cap (if so equipped).

Be careful when removing the bleeding hose. Pinch it right above the fitting and pull it off carefully, so it doesn’t spray fluid. When brake bleeding is finished, and the bleeder fitting is closed, the lever should feel at least as firm as before. Brake fluid should be topped off one final time and the lid replaced securely. Afterward, clean up any spilled fluid and wash the surfaces thoroughly with water. Repeat this procedure for the other brake. Cautiously test ride when done, in a safe area, wearing full safety gear.

Text and photos by Ken Freund


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