Bleeding Motorcycle Brakes

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Let's face it -- bleeding the brakes on your motorcycle can be a pain. But it is a very necessary maintenance item to perform -- and it should be done at least once a year. Brake fluid is hygroscopic (readily taking up and retaining moisture); it attracts water. This water is in the form of moisture in the air. Moisture in your braking system is very dangerous, as water boils faster than brake fluid and can lead to serious problems with the braking performance of your bike. It can also corrode parts of your brake system, including fittings, pistons and the master cylinder. Believe me, I can tell you from experience!

I purchased a 1994 BMW K75 in the winter of 1999. It had only 4,200 miles on it and was a real beauty. But the original owner never rode it much, and other than the initial 600 mile maintenance, had done nothing to the bike. The brake fluid looked pretty brown, so I decided to flush and refill the system. It took about 2 bottles of brake fluid to clear out the system, and then the master cylinder started to leak. I guess the old fluid was so thick, it helped seal the gaskets! I took it apart and found all sorts of corrosion in the bore of the master cylinder, so I had to replace it to the tune of about $175.00. It took about another 2 bottles of brake fluid to flush the system over the next couple of months to get the fluid looking clear and the system working up to par.

Since then, I've gotten into the habit of flushing the brake system, front and rear, twice a year. Once in the fall and once in the spring. I don't put my bikes to bed in the winter; we're fortunate to have occasional days with nice weather where I live, so I try to ride as frequently as I can in the winter. But there can be two or three weeks between rides, and that's when the moisture and corrosion can build up on a bike.

Call me spastic, but I never could get the hang of bleeding brakes by hand. Fill the jar with brake fluid; while holding the jar reach up and squeeze the brake lever (which happens to be on the opposite side of the front brake bleeding screw on my K75); then somehow shut the brake bleed fitting before releasing the brake lever? Or something like that.... Somehow, I just can't see the point of the "speed bleeder brake bleeding valves" sold under various names. You have to put some type of goo on the threads of your bleed fittings, then you still have to use the old "squeeze-release" method. It just seems to make more sense to me to use a vacuum pump for this job.

There had to be a better way. I heard email posters refer to a "Mity Vac" brake bleeding tool. But a friend of mine gave me some good advice about spending the extra few bucks and buying a "professional" system -- the Actron pump and brake bleeding kit. Don't be put off by the list price shown on the Actron site -- you can buy an identical kit but without the fancy carrying case shown on their website for about $45 at Pep Boys or other auto parts stores. It pays for itself in probably one use if you can do-it-yourself rather than bring it into the shop. Shops around here are getting in the neighborhood of $60 per hour, so I figured it would pay for itself with the first bleed.

The Actron unit is very well made -- out of metal. But the best thing about it, I've found, is the vacuum pump gauge. It lets you see exactly what's happening as you perform the bleeding, and as long as the gauge shows that you're holding some vacuum in the system, it means air isn't entering in through the bleed fitting.

Using the Actron Kit It's very easy to use the Actron system and it makes bleeding brakes a simple job with "no muss or fuss". I'm at a point where I can bleed both the front and rear brakes on my K75 in less than 1/2 hour from setup to cleanup. The way I figure it, the easier it is to do, the less likely you'll be to ignore the maintenance!

The complete kit consists of the hand pump, two hoses and a container to hold the used brake fluid. The container has a screw-on top which is airtight once sealed; the top has two nipples on it, one is for the hose that goes to the pump and the other is for the hose that goes to the bleed fitting. There is also a tube of some type of grease you can use to put around the threads of the bleed fitting after you've loosened it, but after I used up the little tube that's supplied in the kit, I simply use regular grease in its place.

Before I start bleeding the brakes, I loosen the top of the master cylinder, and I use an old turkey baster to suck out most of the old brake fluid. Then I fill the reservoir up to the top with fresh fluid -- always put the cap back on the brake fluid container as soon as you can. I also set the top back on the master cylinder, just to make sure no dirt or dust find there way in there.

By the way, here's a tip -- I use a couple of pieces of aluminum foil and spread them around the master cylinder and over the gas tank and anywhere else the brake fluid might drip. Brake fluid is supposed to be pretty corrosive stuff on paint jobs. I also spread some foil around the brake rotor and around the bleed fitting to prevent the fluid from landing on anything important -- like my tire!

Screw the top on the Actron container, attach the black hose from the pump to the container, and the clear hose from the container to the bike's bleed fitting -- the clear hose is so that you can see the fluid as it comes out and you can tell if there are bubbles in it. Pump the handle a couple of times to make sure you can draw a vacuum, and let everything set for a few seconds to make sure the vacuum holds. If the vacuum gauge starts to drop, you probably have some air leaking in, so you should check everything to make sure it's sealed. The instructions tell you to put some grease on the threads of the container to help prevent air leaks, but mine seems to work well without it.

Slightly loosen the bleed fitting, and you should see the old fluid start to come out. I keep about 10 pounds of vacuum going (see the photo on the right), which means you'll have to give a few pumps occasionally. You may want to put some grease around the threads of the fitting to prevent any air leakage. I rest the container on top of an old plastic toolbox, which means this is basically a one-hand operation. It's important to keep an eye on the brake fluid level in the master cylinder -- if I'm alone, when the level gets low, I screw the bleed fitting back in and go around the bike and pour some fresh fluid in. If I can talk my wife into helping me, she keeps an eye on the fluid level and keeps it refreshed.

That's really all there is to it -- keep up the vacuum and you will see the old fluid pour into the container. As long as you keep a vacuum, you know you're sucking fluid down through the system. You can screw in the bleed fitting at any time and see if there's a drop in pressure -- there shouldn't be, but if there is, it means that air is getting into the system somehow and you should check the hoses and the container seal to make sure they're tight. I usually put about 2/3 of a bottle of fresh brake fluid through the system and figure it's flushed. I've never had any problems with air leaking into the system.

Using the Actron pump works great when the system does have air in it, like after you've changed the master cylinder or replaced the brake lines. I'm not sure of the technical reasons, but it seems like when pulling the brake fluid down through the system is better than trying to push it through by hand. The only way that might be better is the professional systems available at some motorcycle dealers that pressurize the system and automatically push the fluid through. This type of system is sometimes used on motorcycles with ABS braking systems.