How to Change the Brake Fluid in a Motorcycle and Why


Hydraulic disc brakes have been around the motor industry for almost 100 years. We had to wait until 1969 though before they eventually found their way on to a motorcycle.

The bike in question was the groundbreaking Honda CB750, and it changed the face of motorcycling forever.

Motorcycle braking systems have come a long way from that humble single front disc brake. Today, the linked ABS systems on modern touring machines are so technologically advanced they rival those found on luxury cars.

Like most components of a motorcycle though, it doesn’t matter how good it is, without regular maintenance it will eventually become a liability. This philosophy goes doubly for disc brakes, forget to keep them in good condition, and things will get nasty real quick.

The Lifeblood of the Braking System:

Apart from changing brake pads and making sure your brake lines are in good condition, there’s one item that often gets overlooked. Hydraulic fluid is the lifeblood of the braking system and needs treating accordingly.

If we use the front brake as an example, brake fluid is stored in a sealed reservoir on the handlebars and connected to the brake caliper via a hose.

When you squeeze the lever, this forces fluid in the hose to move the pistons in the caliper and the resulting friction of the brake pads against the disc is what slows the bike.

Hydraulic fluid, therefore, needs to be none compressible as well as withstanding the type of high temperatures generated under braking. For this reason, most brake fluids use glycol-ether as a base.

Unfortunately, this fluid is hygroscopic which means it absorbs moisture from the atmosphere, hence the need to keep the system sealed. Manufacturers state that brake fluid absorbs around 2-3% moisture per year and advise renewing it every two years.

It’s all in the DOTs:

Most companies adopt the DOT system for brake fluid categories ranging from DOT 3, 4 and 5.1. The difference between them comes down to different boiling points and viscosity. As glycol is the common denominator, it is possible to top up DOT 3 and 4 with 5.1 (as 5.1 has the higher boiling point) but it’s safer to use like for like.

If you were wondering what happened to DOT 5, it’s listed separately as this is a silicone based fluid. Silicone brake fluid absorbs no moisture, and it doesn’t perform as well when compressed.

The result is a spongy feel to the brakes. Incidentally, you cannot mix DOT 5 with any of the glycol-based fluids.

Talking of spongy brakes, this might mean that you’ve got air in the system or the fluid is too full of moisture. Either way, its time to change the fluid, so let’s walk through the steps.

The first thing to do is take a look at your caliper, if you’ve got twin discs, don’t panic; the procedure is the same. Locate the bleed nipple on the caliper and take off the protective cap.

Because of their location front calipers are crud magnets, so a nipple cover is essential. (Helpful hint 1: When you buy a used bike, check out the bleed nipples, if they are rounded off, deduct enough money to allow for having them drilled out and re-tapped).

Right Tools for the Job:

The next thing to do is locate all the items you will need for the job. The list will include plenty of clean rags, a lint-free cloth, and a clear plastic tube that will fit tightly over the nipple. Plus, of course a transparent container, a spanner or socket that fits the bleed nipple perfectly and double the amount of recommended brake fluid.

Here’s the explanation for all of the above items. Clean rags are to cover your gas tank and front mudguard, as brake fluid eats through paintwork like Alien dribble. The lint-free cloth is to wipe away any crud from the inside of the brake reservoir, diaphragm seal and cap thread.

The tubing has to be clear, so you can see the progress of the brake fluid and look for air bubbles. It also has to be long enough to fit on the bleed nipple and curve down into the container, without any bends or flat spots.

Like the tube, the container needs to be clear so you can check for air bubbles as the brake fluid pumps into it.

Buy More Brake Fluid than you Need:

The spanner to undo the bleed nipple has to be an excellent fit because OEM bleed nipples are easily damaged. (Helpful hint 2:  Buy an aftermarket bleed nipple in stainless or titanium). Finally, why double the amount of brake fluid? Because you always use more than you think, especially if you cant get rid of air bubbles.

With everything assembled, its time to proceed, and the first thing to do is to put the rags over your tank (Helpful hint 3: If you do spill brake fluid on the rags, remove it before it can soak through to the tank).

Next undo the bleed nipple a half turn. Open and close it a few times to make sure it’s moving freely, then push the clear tube over it and put the other end into the clear container.

Taking a rag, clean the outside of the reservoir on the handlebars. Undo the cap and with the lint-free rag clean the thread of the cap. Next,  take out the diaphragm, inspect it for holes, then clean it and wipe around the inside of the reservoir above the fluid level.

Pump the Lever Slowly:

The bleed process is next, so back off the bleed nipple a third of a turn to allow the fluid start to flow.  Pull the brake lever in slowly to accelerate the flow of fluid through the system.

With the lever still pulled in and fluid flowing, re-tighten the bleed valve. (Helpful hint 4:  If you have a twin disc system the process is the same, just start with the caliper furthest away from the reservoir).

Check the reservoir and top up with fresh fluid, then repeat the process of opening the nipple, slowly squeezing the lever and re-tightening. (Helpful hint 5:  Be careful not to fill the reservoir too much, or pull the lever in too quickly, either could result in paint-eating brake fluid spurting or overflowing).

When only clean fluid remains in the reservoir, and no air bubbles are visible in the tube, tighten the bleed nipple and replace its cover.

Check the level of fluid in the reservoir, making sure it falls between the ‘upper and lower’ lines, and replace the diaphragm. Finally, screw on the cap and test the lever pressure.

How to Change the Brake Fluid and Why?

Your brake system is full of clean fluid; you’re ready to ride, and now you know how to change the brake fluid in a motorcycle and why.