Sept. 3, 2016 | By Jeff King
In this month’s tech article, we’re going to cover the function and servicing of the hydraulic braking systems on your motorcycle.
By better understanding how these systems function, it will be easier to keep tabs on the health of your brakes, and perform the yearly maintenance procedures to keep them working their best.
A hydraulic system is defined as any system that uses liquid pressure (in our case, brake fluid) to generate or transfer mechanical power. As liquids are incompressible, any pressure applied to one end of the system will be transferred directly and efficiently to the other side.
Here’s a diagram of a simple hydraulic brake system:
The brake pedal (or brake lever) is attached to a brake master cylinder. The master cylinder contains a small reservoir of brake fluid and a master cylinder piston that is used to exert force on the hydraulic fluid when the brake lever is depressed.
Pressure applied by the brake lever to the master cylinder is transferred down hydraulically through the brake lines to the brake caliper.
In the brake caliper, the brake fluid acts on another piston (the brake caliper piston), pushing the brake pads up against the disc brake and provide braking power.
So what can go wrong, and what do we need to look out for?
We’re actually asking a lot from our brake fluid. Because of the heat generated by all that braking friction, we need our brake fluid to handle the high temperatures generated during heavy braking and still give us consistent performance. Brake fluids are specifically formulated to have a high boiling point (over 200 degrees), to avoid vaporizing (boiling) in the lines. Once vaporized, your brakes become less effective, as liquid vapor is very compressible. Any pressure applied at the brake lever goes into compressing the vapor, not pushing the pads against the disk.
Brake fluids are hygroscopic fluids, meaning they are susceptible to absorbing moisture from the atmosphere. Even though your brake system is a closed system, atmosphere and humidity do get in the system, and eventually contaminate the fluid enough to effectively lower its boiling point. That spongy feeling at the lever during prolonged heavy brake use is a sure sign that your brake fluid is contaminated with moisture.
It is for the above reasons that it is recommended that your brake fluid be changed (flushed) yearly, to keep fresh, uncontaminated brake fluid in the system, and assure the highest level of performance from your braking system.
Brake fluid comes in several different flavors (DOT 3, DOT 4, DOT 5 and DOT 5.1). The DOT stands for "Department of Transportation", the governing body that sets the specifications for brake fluid performance.
DOT 3 and DOT 4 are very closely formulated (glycol based), so much so that they are essentially interchangeable, often referred to as one fluid ... "DOT 3&4". DOT 5 has a significantly higher boiling point (260 degrees), but is a silicone-based fluid. DOT 5.1 is glycol based like DOT 3&4, but has a much higher boiling point and is used for higher performance braking systems.
The important takeaway here is that your braking system is designed to use a specific formulation of brake fluid, and with the exception of DOT 3&4, they are not interchangeable. The rubber seals in your braking system are subject to damage (swelling, deterioration) if the wrong brake fluid is used.
Crack open a bottle of your favorite brake fluid and note its color. You’ll see that fresh brake fluid is almost clear. (This will probably be a stark contrast to what’s currently in your master cylinder reservoir ... aged brake fluid often takes on a coffee color.)
DOT 3&4 brake fluid is by far the mostly commonly used, but there are exceptions. Almost all brake master cylinders have their specifications stamped into the reservoir cap (like "Use only DOT3&4 brake fluid"). Check your service manual if there is any doubt.
Fresh brake fluid
Rags or paper towels
8mm or 10mm box end wrench
Vinyl tubing & Drain Pan OR
One Man Bleeding Kit (I really like this little number from Harbor Freight)
Bleeding the brakes
Set your bike up on a nice level surface.
If you have access to a bike lift or have a centerstand, use it! Although this procedure can be accomplished on the sidestand, you’ll have an easier time if the bike is held steady and level.
At the handlebar, remove the cap to the reservoir on the brake master cylinder.
There’s often a rubber gasket or two under the cap, take note on their location and order so you can easily put it back correctly when we are done.
Down at the brake caliper, locate the bleeder bolt.
Bleeder bolts are often 8 or 10mm in size, but check yours and make sure you are using the appropriately sized wrench for the job. Set the wrench on the bleeder bolt first, and then attach the vinyl tube to the bleeder bolt like this:
Run the vinyl tube into a container to catch the old brake fluid. Make sure and top off your master cylinder reservoir with fresh brake fluid.
OK, now that we’ve set the scene, we get to the meat and potatoes of the service. The goal here is to push out the old brake fluid, and cycle in the fresh fluid without drawing any air into the system.
Here’s the order of operations:
1. Open the bleeder bolt at the caliper a 1/4 turn (counter clockwise)
2. Squeeze the brake lever (or pedal)
3. Close the bleeder bolt (clockwise)
4. Release the brake lever
As you squeeze the brake lever, you’ll see brake fluid exiting the caliper out the vinyl tube. As the fresh brake fluid replaces your old, the color of the fluid will change from dark to light as well. If there is any air in your brake lines, the vinyl tubing also helps you identify any bubbles in the fluid.
Repeat Steps 1 through 4 until all your old fluid is flushed out, and you only see fresh brake fluid (with no bubbles) exiting the caliper.
During the process, keep an eye on the level of brake fluid in your master cylinder reservoir. After 5 to 10 squeezes or so, you’ll likely need to top off with more brake fluid. If you happen to let the reservoir run dry, your next squeeze of the brake lever will likely pump air into the system, and you’ll have to start the process all over again. If that happens, no worries, you are in good company. It happens to everyone. Just take your time and start the process over again, paying closer attention to the level in the reservoir.
When you see only nice clean brake fluid exiting the caliper with no bubbles, snug up the brake bleeder bolt and test the feel at the lever. The brake lever should feel firm, not spongy.
If the brake lever still feels spongy, there likely still air in your system. Continue the bleeding process until you get all the air bubbles out of the system, and you have a firm feel at the lever.
Once you have a firm feel at the lever, it’s time to button up! Make sure and top off the reservoir with fresh fluid up to the fill line, fit the rubber gasket and reservoir cap back on the reservoir.
Nice work! You’ve assured top performance out of your brakes and completed the yearly service.
If you’d like to get some real world experience on this procedure, Moto Republic offers a "Brakes 101" workshop monthly, and private one-on-one instruction is available as well. So come on in and get your service on! Check out our current Workshop and Class listings here on www.moto-republic.com.
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DISCLAIMER: The methods in this article are guidelines, provided and performed by a trained mechanic. If you are not comfortable performing routine maintenance on your bike, it is recommended you get a trained mechanic to complete these and more difficult tasks on your bike.
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