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How-To Flush and Bleed Motorcycle Brakes // AJAX Tech Tips

Posted By: Ajax Motorsports
Post Date: 09/14/2017


A commonly overlooked segment of bike maintenance has to do with brake fluid.  Most people don’t understand that brake fluid needs to be flushed and changed periodically. If they do know, they often avoid the task like the plague because it can be frustrating.


Why must brake fluid be changed?

Brake fluid is hydroscopic, meaning it has the ability to attract and hold water.  This water doesn’t function the same as the fluid and decreases the boiling point which leads to spongy brakes once they heat up. The brake fluid also breaks down over time and loses its effectiveness.


What are the differences in brake fluid?

DOT 3, 4, and 5.1 are commonly used in motorcycle braking systems. The higher the number, the higher the heat rating. Brake fluid, when too hot loses its ability to transmit hydraulic pressure and leads to brake fade. Under hard use, the brakes won’t work at all. This is commonly referred to as “boiling the brakes.” A higher number means the fluid is resilient to a higher temperature and better for extreme braking. We always recommended to use the type of fluid recommended by your owners’ manual. You may elect to use a higher rating of brake fluid, however, do not use DOT 5 (not to be confused with 5.1) because it is silicone based and isn’t compatible with most motorcycle brake systems.


What about Mighty Vacs or syringes?

Many people choose to go this route and it’s fine if you do too! Just be careful because these methods are very effective at pulling a vacuum and can work too quickly on these small systems. We prefer using hose and gravity. It may take a little longer but you won’t have to redo the work when you suck air back into the system.



Basics: When flushing or bleeding brakes, your enemy will be air pockets trapped in the line. Sometimes getting them out is easier said than done.

1.     Put the box end of the wrench on the bleeder then attach the hose.

a.     If flushing only, you can put the other end of the hose in a bottle to catch the waste fluid. If bleeding, use a longer hose that can be elevated above the master cylinder.

2.     Open the reservoir cap and clean.

a.     If flushing: Remove old fluid with a sucker or syringe and scrub sediments. Add a little clean fluid to wash it out then remove that as well

b.     Clean the cap and billows to remove dirt and moisture.

3.     Top off the reservoir and make sure it stays that way.

a.     If the system gets too low and sucks air, you’ll have more work. So keep an eye on the level above all else!

4.     Crack open the bleeder.

a.     If flushing, direct the waste fluid to a bottle or container.

b.     If bleeding, elevate the hose above the master cylinder. This will ensure the air escapes by equalizing pressure.

c.     Note: typically, dirt bike front brake lines rise above the master cylinder for the first six inches or so. This is a prime place for air to get trapped. Consider pulling the line down and securing it with a zip tie to the fork leg. Also, turning the bars to the left can elevate the master cylinder.

5.     Flushing

a.     Flush about two reservoirs or more through the system to ensure you have fresh fluid in the system.

b.     Pump the pedal/lever to force fluid through the line.

c.     Be sure to keep fluid levels full as to not contract air.

d.     Once finished, close the bleeder and pump the brake to ensure it feels right. The feel should be firm. If it feels spongy, there is probably air in the line and needs bled.

6.     Bleeding: Let gravity work.

a.     Especially on front brake systems, when using the hose method, gravity does most of the work for you. Just keep fluid levels full and watch as the fluid makes its way from the caliper up the line to above the master cylinder.

i.     As the fluid level in the bleed line nears the master, pressure begins to equalize. This is when you’ll notice the majority of the air come the the surface.

ii.     Air can escape either through the bleeder line or back up through the reservoir.

 iii.   Tap the line to free air bubbles and work them to the surface.

b.     Pump the lever/pedal if you wish to expedite the process by forcing fluid through the line. Just be prepared to add fluid.

c.     Once you notice air bubbles are no longer coming out, close the bleeder and pump the brake to verify a solid feel. It is not uncommon to have small bubbles still working their way up through the master cylinder. Once this stops, top off the level just below max and replace the cap and bellows.




Final tips:

1.     Don’t open the bleeder too much. Crack the bleeder slightly to allow air to escape but not so loose to allow air past the fitting.

2.     Brake fluid will harm painted surfaces. Be careful where it ends up.

3.     If you accidently suck air in, don’t sweat it, just try again.



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