Changing The Brake Fluid

Originally posted to El Cantar de la Lluvia on Sunday, March 25, 2007

Last night I changed my motorbike's brake fluid for the first time. It is extremely easy; an insignificant error does not cause giant problems.

Why did I decide to do this?

Let's see: My bike is a '96 XR250R, and I have no reason to believe the brake fluid has ever been changed. My objective was to achieve better braking, in case air bubbles had appeared in the system, and to change the fluid, something that must be done every now and then.

But why is this necessary?

First, a little bit about brake fluid. There are two types: glycol based, and silicone based. Glycol is what you use in your radiator as an antifreeze. It's a fluid that's slightly more viscous than water, is transparent, and is polar, which means it mixes with water.

Silicone is does not mix with water.

Brake fluids, according to the American designation, are labelled as DOT 3, DOT 4, DOT 5 and DOT 5.1.

DOT 5 is the only one of the series that is silicone based, and is generally purple or blue. The rest are transparent and glycol based.

Brake fluid transfers pressure applied at the brake pump to the brake caliper. If there were a bubble of air in the circuit, the pressure applied would be used to compress the bubble, and not to press the brake pads against the disc.

There are two sources of bubbles: air and water vapour. Water vapour is the one that becomes more relevant as times goes by, since glycol is hygroscopic, that is, it absorbs humidity from the environment.

"But brake systems are sealed!" I hear you say. This is not so: though the brake fluid reservoir has a rubber membrane (beneath the metal cover) that allows the brake fluid level to rise and fall without coming into direct contact with the environment, this piece of rubber, and all other pieces of rubber in the system, allow for a very gradual penetration of humidity. It isn't much, but over several years, it is a considerable effect.

At room temperature, the water dissolved in the brake fluid has no effect, other than to contribute to the corrosion of the pistons, pump and other elements of the brake system.

When you brake aggressively or for extended periods of time, the caliper heats up, and the brake fluid's temperature rises also.

The effect of water dissolved in the brake fluid is to lower the mixture's boiling point. Note that just 1% water by volume considerably affects the boiling point, lowering it noticeably. I won't give specific data here, but you can find it easily enough.

And what does this mean? That if there is enough water dissolved in the brake system, the fluid might reach its boiling point under aggressive braking, producing vapour bubbles immediately.

Therefore: we we wish to avoid this. This is where the DOT ranking is useful. DOT 3 has a lower boiling point than DOT 4, DOT 5 or DOT 5.1. That's why DOT 4 is better than DOT 3.

And what about DOT 5? The good thing about DOT 5, since it is made of silicone, is that it does not absorb water. And the bad thing about DOT 5 is that it does not absorb water. Yes, you read that correctly. Since it does not absorb water, any humidity that penetrates the system will accumulate into droplets, generally at the lowest point, the caliper. This will lead to corrosion, and will increase the risk of malfunction when braking aggressively.

Another negative aspect of DOT 5 is that it aerates easily. This means that if you shake the bottle, the liquid will become full of suspended tiny bubbles, which will not disappear until hours later.

Also, brake line seals designed for use with DOT 3/4 may not be adequate for use with DOT 5.

And finally, DOT 3/4 should never be mixed with DOT 5, as this would be like mixing oil and water.

And DOT 5.1? It is a brake fluid with a similar boiling point as silicone based DOT 5, but made from glycol. It is my experience that DOT 5 can be some 4 times as expensive as DOT 3 or 4.

Ok, now we know why we need to change our brake fluid every now and then, and which one we should use. In my case, that would be DOT 4.

But how do we do it?

Look at the caliper. You'll probably find a small rubber cap. Remove it: underneath, there is a nipple suitable for connecting a tube. Buy cheap fuel line hose at some home maintenance store. It should be transparent. Connect it to the bleed valve.

On the base of this nipple there is a hexagonal nut. With a small wrench, probably an 8 mm, one can open the bleed valve with 1/8 of a turn. We will do this later on.

Let's look at the brake fluid reservoir.

First, it is convenient to protect plastics, paint and rubber around the reservoir with wet paper towel, in case of a spill.



Second, remove the metallic cover, the plastic cover, and the rubber diaphragm. In my case, the diaphragm had a rather strange deposit on the inner surface.

Since we want to get rid of the old brake fluid, we absorb it all with some paper towel, without touching the brake lever.



Once the reservoir is empty it can be filled with fresh fluid. This first load will serve to purge the old fluid from the system.

Changing your brake fluid will probably require help from a friend. We'll call him B, you're A.

1. A pumps the brake lever a few times. On the third pump, it is kept pulled back. Say "Open".
   
2. B gently turns the nut on the bleed valve until liquid starts flowing into the transparent tube.
   
3. When A feels the brake lever give, and when it is about to reach the handlebar, say "Stop".
4. B closes the bleed valve nut.
5. A lets go of the brake lever, noticing how the level in the reservoir goes down, and pumps the lever a few times. On the third pump, it is kept pulled back. Say "Open".
   
6. B opens the bleed valve nut.
7. A feels the brake lever get close to the handlebar. Say "Close".
   
8. Etc.
   

This cycle is repeated until the level in the deposit has gone down considerably. Without allowing it to run dry, more brake fluid is added.

When there are no more bubbles or dark brake fluid coming into the tube, the bleed valve can be finally closed, the reservoir can be filled up to the mark, and the whole system can be closed up.

The back brake is easier, since the pedal and the reservoir are quite close to each other.

And that's it.

In my case, the old fluid was the colour of Coca Cola, and it was noticeably more dense than the fresh brake fluid (you could see the dark drops sinking to the bottom of the reservoir when I added the new fluid). I have read that the fluid's darkness is not always an indicator of a dangerous level of water content, but if it's dark, why not change it?

So now you know: there is no reason not to change your brake fluid every year.