How to Flush Your Brake Fluid

How to Flush Your Brake Fluid

As we’ve discussed recently, your motorcycle’s stopping power is probably more important than its “go” power. Your braking system is actually more of a speed regulator than even the throttle itself. Therefore, you need to flush your brake fluid every year or so for your bike to be in excellent working order.

Bleeding your brake reservoir allows you to easily phase a fresh supply of fluid into your braking system while eliminating any air bubbles. If, however, you want to ensure maximum braking efficiency by completely removing old fluid and then renewing it, then you’ll need to perform a full flush. Thankfully, the procedure is basically the same, though the flush will take you a little longer, as it is more thorough. Once you check your motorcycle’s service manual for the correct brake fluid maintenance interval, we can move on and take a look at how to perform this simple, yet essential maintenance task.

Things you will need:

  • New brake fluid (of the correct type)
  • Clean shop towels or rags
  • Screwdriver (if your brake reservoir/master cylinder cap is held on by screws)
  • Wrench (preferably box end)
  • A glass jar or plastic jug with plastic tube
  • A vacuum type brake bleeding kit
  1. First, you’ll need to cover up your fuel tank and any other painted surface that is in danger of being dripped on with your clean cloth. Brake fluid is generally very effective at removing paint, so you want to protect yours. Additionally, wrap the master cylinder in cloth to catch any direct spill or overflow. Any spill or overflow needs to be removed immediately.
  2. Remove the cap from your brake reservoir. There will be a diaphragm directly underneath your cap. Remove that as well, then set the two items aside carefully. The brake fluid will now be exposed and you will be able to pour more in as you need to.
  3. Find the bleed nipple bolt on your brake caliper and remove the cover. Place a box end wrench of the appropriate size on the bleed nipple bolt. Next, place one end of your plastic tube on the bleed nipple and the other end in your jar or jug.
  4. Loosen the nipple anywhere from 1/3 of a turn, but make sure you don’t loosen it so much that the nipple bolt comes out, allowing the fluid to escape from the caliper.
  5. A) If you are bleeding your brakes, squeeze the brake lever gently and watch carefully as the fluid empties out into your receptacle. Before the fluid drains completely, tighten the nipple bolt, then release the lever. Refill the reservoir and loosen the nipple bolt before squeezing the brake lever again. Repeat this until you’ve eliminated all air bubbles and your brake fluid is all new.

    B) If you are flushing the system, squeeze your brake lever repeatedly until all the brake fluid has been emptied out into your receptacle. Once you are sure there is no fluid remaining, tighten the bleed nipple back up. Refill the reservoir. Go back to step 4 above, then proceed through step 5A. Be sure not to drain the reservoir completely and keep adding brake fluid as you go along. Keep going until there is no visible air and until the firm feel is restored to your brake lever.
  6. Tighten the bleed bolt, remove the plastic tube, and remove the wrench. Replace the bleed nipple cover.
  7. Carefully replace the diaphragm, and then the reservoir cap.

If you have a vacuum type brake bleeder at your disposal, your life will be easier. It will include its own reservoir into which its plastic tube will feed, for one. Simply attach that plastic on the bleed nipple, as per the end of step 3. Pump the bleeder a few times until you have adequate vacuum built up. Now, loosen the bleed bolt and the fluid will be sucked right into the bleeder’s reservoir without the need to squeeze the brake lever. You can also use the vacuum pump to thoroughly suck new fluid into the system, but remember to keep topping up your brake reservoir to keep from emptying it again at this point. Once you are satisfied that your braking system is adequately filled with fluid and that you have the right brake lever feel, then proceed from step 6 and button it up. Vacuum pumps are much more effective than the basic plastic tube and jar.

The Haynes Motorcycle Maintenance Techbook by Keith Weighill offers an alternate method for re-introducing brake fluid into a completely flushed system called “back bleeding.” This entails feeding the brake fluid up through the caliper. This method requires you to fill a new oil can (it must be new) ¾ full with brake fluid. Fit a plastic tube to the spout of the can, and then prime the oil can by pumping the lever a few times. Place the other end of the plastic tube on the caliper’s bleed nipple, loosen the bleed bolt, then pump the oil can. Keep pumping until the brake reservoir level starts to rise. Pump slowly until the reservoir is full, then proceed from step 6 above. This method is more for when you fit a new brake line, though most will probably opt for the vacuum pump in any event.

Again, this is a pretty simple maintenance operation, but among the most important. Ultimate braking efficiency should be your goal, and a thorough brake fluid flush is a great way to ensure that your braking system is in top condition. This will give you the confidence in your brakes that you want while maximizing the safety that you need.


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