Tech Tips

How to Bleed Hydraulic Brakes

Written by  February 28, 2005

In my last article, we conquered fork seal replacement, and now it is time to address that dirty old brake fluid you see in the window of your brake master cylinder window or hydraulic clutch reservoir.

For many, this task is one of those items, like the saying goes, “if it is not broken, don’t mess with it.” I on the other hand prefer to replace contaminated fluid as part of my regularly scheduled maintenance. I have asked my friends and colleagues how often they think they need to change their brake or clutch fluid. The answer I usually get is every three years.

Let me shed some light on this. Brake fluid is used in the hydraulic clutch and front and rear braking systems of most late model motorcycles in production today. When the fluid breaks down, the brake pistons have a tendency to stick and cause your brakes to drag and makes it hard to pull your brake lever. The reason your brake fluid should be changed more often than you realize is because these systems are in constant stress from the brake hoses, hard braking, and using the clutch in traffic. This fluid can also draw moisture into the system. Therefore, I recommend that you change your brake fluid every one to two years, minimum. The bike we are servicing today has less than 300 miles on it and is a 2003, and by the color of the fluid, I can see that it is already starting to break down after just two short years.

We have more options available today, than in the past to bleed these systems more safely and efficiently. You must be very careful when changing your brake fluid because it can take the finish off your paint in only two to five minutes, and plastic parts can be ruined just as fast. The handlebar control boxes usually get the blunt of the fluid, which is located next to the brake or clutch reservoir. One of the best pieces of advice I can give you is to cover your motorcycle with a large towel or moving blanket before removing the top cover of the reservoir. This prevents spilling even the smallest amount of brake fluid on your paint job. Another tip is to spray WD-40 on your control boxes. This helps neutralize the fluid and makes the plastic nice and new looking, even if it has been faded by the sun. Now that we have discussed the precautions of performing this job, let’s get the old fluid changed and our bike back on the road.

The first thing you need to do is identify the type of brake fluid you need. It is either Dot 3, 4 or 5. If your system requires Dot 5, like this 2003 Sportster, you want to use Dot 5. Dot 3 and 4 is for earlier model motorcycles and works fine with most late 80s and early 90s models. The easiest way to determine the type of brake fluid you need is to look on the top cap of the reservoir. It usually tells you the type of fluid required. I do not recommend using a different type of fluid to cut corners. If in doubt, always refer to your owner’s manual or service manual for the type of fluid your bike requires.

The type of tools required for this procedure depends on the size and type of the brake bleeding screw on the caliper. Most metric models use an 8 or 10mm wrench, while most American models require a 5/16 or 3/8” wrench. If you don’t have the appropriate wrenches for this procedure, just run down to your local auto parts stores. And while you’re there, this would also be a great time to buy new brake fluid. I know that most of us probably have a bottle of brake fluid sitting around in our garage, however, I want to stress that an open bottle of brake fluid tends to draw moisture after awhile. Therefore, you should use only new brake fluid when performing this procedure.

Now that you have your bike covered, it is time to open the master cylinder reservoir. Normally, this requires a #2 Philips screwdriver unless it has Allen bolts. Screws can sometimes be stubborn, so be careful not strip the head of the screw. If you strip a screw you will have to drill the head off and purchase new ones. Therefore, if you encounter any stubborn screws, you might need to remove them with an impact driver. This is a tool that not everybody may have, so to avoid stripping any screws, it may be worth a trip to the parts store or your neighbor’s garage.

Remove the top cover and you see a rubber diaphragm and a plastic spacer that is usually located above the diaphragm. Take a look at the fluid, and if it is brown and jelled looking, you need to remove all of the contaminants because these cannot be removed by bleeding.

The actual bleeding procedure takes only a few minutes and there are a couple of ways this can be accomplished; Gravity Bleed and Pneumatic Assist. I prefer the Pneumatic Assist method because it works great with ABS-type systems. When purchasing one of these systems, only buy what your budget allows. I have been doing this for some time now and need a system that is fast and efficient. If you wish to use an off-the-shelf Mityvac, it can be purchased at your local auto parts store or at They are inexpensive, can be used again and again, and work with just about any make of brake system.

If you are using the Gravity Bleed method, take an old plastic container, Mason jar or pop bottle, run a piece of plastic hose from the bleeder screw to the bottle, and this serve as the collection point for the old fluid. You can have a buddy help poor the new fluid in the master cylinder as you open the bleed screw and pump the lever. This forces the old fluid out of the system and keeps the master cylinder primed. You have to watch the level of fluid because you do not want this get too low and have air enter the system. If air does get into the system you have to start this process all over.

Before the Mityvac days, we had to do this by hand. It was very messy and hard to get the air purged from the system. Air in the system gives you a spongy lever and a pulsating feeling when the brakes are applied, which is not what you are looking for when you finish this procedure.

When using the Pneumatic Assist method, you are pulling a constant vacuum through the system and it completely drains the system in record time. That’s why I like to have a buddy help fill the reservoir while I am at the caliper with the tools. Using both types of bleeding, you are looking for clean new fluid to come from the bleed screw. You can check for brake pressure at any time by simply closing the bleeder screw and pumping the brake lever. If it feels solid you are on the right track. You may see air bubbles from the bleeder screw, but that’s OK, because you want the air and debris out of the lines.

Once you have bled the system and clean fluid is present with no air in the lines, top off the master cylinder reservoir and reinstall the diaphragm and top cover.

Once you have done a brake system, a clutch system is the same basic procedure, and should be performed every one to two years as well. If you take your time and keep safety in mind, a beginner can accomplish this procedure in an afternoon. This procedure can be fun and rewarding, while saving you money.

Our Tough-O-Meter scale ranks this at a 3 on a scale of 1-10, so just about any body should be able to handle this. If you have questions, or are uncomfortable with this procedure, please e-mail me, because I am here to help!

B-safe out there!

By Dave Miller