Make sure you can stop that thing:

Brake Bleeding 101

by

Judy

I. Background
Having your motorcycle’s brakes functioning properly is something your life depends upon. When was the last time you looked at them, or had them checked by a qualified mechanic?

Modern bikes typically have disc brakes front and rear. Some older or smaller bikes may still have a drum brake on the rear, but even they most likely have a disc up front. How can you tell?

Look at your wheel. If you see a big shiny round disc with holes in it attached to the wheel, then you have disc brakes. (Figure 1).This article will focus on the maintenance of disc brakes, since everyone likely has them on at least their front. (…and, we do know that at least 75% of the motorcycle’s stopping power comes from the front brake, and we use it routinely, correct?)

Some background information: Drum brakes, popular prior to 1969, were operated mechanically, usually by a cable that was activated (like your clutch cable) by a lever or pedal, which caused brake shoes to press out against a cylinder (located in the axle), causing friction to stop the bike. Disc brakes, on the other hand, are operated hydraulically, having components that include brake fluid which hydraulically activates the pistons, which push the brake pads against the rotor (the big shiny disc described above), causing friction to stop the bike. The big disadvantage of the old drum brakes was that they generated a lot of heat. As the heat builds up, the effectiveness of the braking power decreases. So, they aren’t a good choice for high horsepower or heavy motorcycles, hence the switch to disc brakes. Enough for the history lesson, and onto a description of disc brakes and how to maintain them.

The disc braking system has several components. At the top is the master cylinder, on your right handlebar. It consists of a cylinder and piston assembly, and is attached to the brake lever. The other component of the master cylinder is a reservoir containing brake fluid. (Figure 2).

It will either be a whitish colored cup (Figure 3), or a black or chrome metal reservoir with a site-glass. (Figure 4).

Inside the cap on this reservoir is a rubber diaphragm. Figure 5 shows proper placement. Figure 6 shows improper placement. Attached to the master cylinder are the brake lines, which contain brake fluid. Follow them down towards the wheel,and you will see that they attach to calipers. The calipers fit over the rotors (Figure 7). Inside the calipers are pistons. There are also brake pads. Each caliper has two brake pads. The caliper fits over the rotor (the big shiny disc with holes in it).

 

There is one brake pad on each side of the rotor. Figure 8 shows new brake pads in the packaging. Figure 9 shows the new brake pads from a side view. Figure 10 shows the caliper removed from the rotor.

Figure 11 shows a side view of the removed caliper, showing the pistons in the caliper and the wear groove indicator in the brake pad. When you pull your brake lever, the master cylinder piston moves, pushing the fluid through the lines, creating pressure which then pushes out the pistons in the caliper. The pistons press the brake pads against the rotors, producing friction to stop the motorcycle. Heat is produced, but the calipers are open to airflow (unlike the enclosed drum brakes), so the heat can dissipate. This is a fairly simplistic description, but it gives you a general idea of how the brakes work.

The brake fluid is what you’ll need to change periodically, usually about once a year. Check your owner’s manual for a maintenance schedule. Brake fluid is a mineral oil that has been designed to resist boiling. One disadvantage of brake fluid is that it’s hygroscopic, meaning that it readily absorbs moisture from the surrounding air. When it does this, the boiling point is lowered, so the fluid can overheat, resulting in poor brake performance. This is the main reason it needs to be replaced regularly.

When replacing your fluid, always replace it with the recommended type, usually DOT 3 or 4. Your owner’s manual will state which fluid you’ll need, and it’s usually stamped on the reservoir cap as well. DOT 3, 4 and 5.1 are interchangeable. NEVER use DOT 5 in a system that calls for DOT 3 or 4!! Using the wrong type of brake fluid can cause brake failure.

II. The Procedure
1. Check the level of the fluid in the reservoir, ensuring that the fluid is between the min and max lines. Check the color of the fluid. The color should be amber, about the color of a Molson beer. Once it starts getting dark, it needs to be changed. If it’s dark like the color of used motor oil then it’s contaminated and should be changed ASAP. This is a quick visual check that should be done often, when checking tire pressures and such. Also check the brake pads for wear at this time (I won’t go into changing pads in this article). The brake pads have wear grooves etched in them. When the pad has worn down and is even with the groove, the pads need replacing. (See Figure 11). To see the pads, look inside the calipers from the front of the bike. You’ll need a flashlight to get a good view.

2. Get several old rags or towels and cover your handlebars, instrument cluster, and wheels. Brake fluid is very caustic, and will quickly ruin paint and crack plastic if it’s spilled. If you do spill some, even a small amount, immediately flush with copious amounts of soapy water. I recommend wearing a pair of nitrile gloves to keep brake fluid off of your skin.

3. Remove the reservoir cap. Ensure the diaphragm is tucked up inside, not stretched out. (Figure 5)

4. Remove the rubber cap on the bleeder nipple, located on the caliper. (Figure 12) Attach a piece of clear tubing to the bleeder nipple on the caliper, and place the other end in a waste container. (Figure 13) An old soda bottle will do. Label it as waste brake fluid. Be sure is has a screw on cap. Save it and dispose of it properly, at your town’s hazardous waste material collection day. Do not pour it down the drain, or on the ground, it is toxic.

5. Open the bleeder nipple about ½ turn, using the appropriate size wrench, usually an 8 or 10 mm.

6. Squeeze the brake lever all the way in. DO NOT release it yet.

7. Close the bleeder nipple. Then release the brake lever.

8. Repeat steps 5 – 7 enough times to replace the volume of fluid in the reservoir about 1 ½ times. Keep a close eye on the level in the reservoir, so that it doesn’t run dry!! If it does, it won’t hurt anything, but you’ll have introduced air into the system that you’ll need to bleed out. Like moisture, air in the lines will decrease the braking power. Add fluid to the reservoir as necessary. I pour the fluid into a measuring cup with a pour spout, as you’ll be less likely to spill. Pouring directly from the bottle will result in spilling and dripping.

9. While bleeding, observe the fluid coming out of the clear tubing. At first, you will probably see air bubbles coming out. Keep bleeding until bubble-free fluid is coming out. (Figure 14)

10. When there is no more air in the tubing, snugly close the bleeder nipple and carefully remove the tubing, and replace the rubber cap.

11. Top off the reservoir to the correct level and replace the cap.

12. Pump the brake lever until it is firm.

III. Conclusion and Additional Tips
Some bikes have two discs in the front, some have only one. If your bike has two, start bleeding with the one on the right (as you are sitting on the bike). When no more air is seen in the tubing, then bleed the one on the left.

If your brakes still feel “mushy” you can gently tap on the brake lines with a screwdriver handle to dislodge bubbles, then repeat the bleeding procedure. Another trick to get a firm lever is to wrap a bungee around the brake lever, holding it on overnight, then repeat the bleeding procedure the next day.

If you’d really like your brakes to feel extra firm, consider replacing the stock rubber brake lines with braided steel lines. As pressure is introduced into the brake lines when the lever is squeezed, the rubber expands a bit, reducing the force of pressure delivered to the calipers. Stainless steel lines, on the other hand, will not expand, so all the force is delivered to the calipers.

Be patient! It can take quite awhile to get the air out. If there is a lot of air, and you can’t get the fluid to start coming out of the lines into the clear tubing (as is usual if you have bled the system dry to replace lines) you may need to use a MityVac® , or some other vacuum system to get the fluid flowing, then do the bleeding procedure described above.

If the Club members have any questions or would like a demonstration, feel free to email or call me, or talk to me at the next meeting! If there is enough interest, I’d be happy to have a little seminar on this and other maintenance topics.

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