How to change rear brake fluid on a motorcycle

Changing your brake fluid – it has to be on the DOT.

Getting that fresh fluid feeling
Hydraulic brakes – that’s virtually all disc brakes, which covers 90 percent of modern bikes – theoretically operate as a sealed system. Squeeze the brake lever and fluid in the brake line pushes against the back of the pistons in the brake caliper, which push against the brake pads, which bite the disc, which slows you down. Simples, right? In theory, yes.

In practice, there are some very important points that make all the difference:

• The system isn’t 100 percent sealed – there’s some scope for air to intrude
• To a greater or lesser extent, air contains water
• Brake fluid is ‘hygroscopic’ – it actually attracts water
• After some time, your old brake fluid will contain a small amount of water
• Down at the pads, braking friction means things get very hot. Hot enough to boil any water in your fluid
• Boiling fluid generates steam pockets that can be compressed, causing your brakes to fade, or go away completely.

So if your brakes, once strong and positive, become soft, vague and spongy, it’s very likely due to old fluid, worn brake pads or both. 

You know that feeling when the lever starts coming back to the grip, you adjust the lever and it’s soon back to the grip again? Or when you go for a ride and the brakes start out feeling great, only to fade as you use them more and more? Very often, that’s old, shitty brake fluid taking its toll.

What brake fluid do you need for your bike? Check twice, use once.

Keep an eye on the colour
It’s a good idea to refresh your brake fluid regularly regardless of kilometres travelled, but one sure giveaway that it needs changing is a noticeable difference in colour. Many brake fluid reservoirs are transparent; others have a little viewing window so you can see the fluid – its level and its colour. If fluid that started out clear, amber or green starts to look murky and dark, it’s contaminated and overdue for a change.

DOT ratings – the details matter
On the cap of virtually all brake fluid reservoirs you’ll see an instruction that dictates that you use only a certain ‘DOT’ rated fluid. So what the hell is that all about? Isn’t all brake fluid the same?

Nope.

The DOT rating system is the brainchild of the U.S Department Of Transport. Your whole braking system is designed to work with just one type of fluid. No matter what Johnny Racer Boy tells you, a higher DOT number is not better. The best DOT fluid for your bike is the one that’s called for on the cap of your brake fluid reservoirs. Likewise, brake fluid labelled ‘race’ is not better fluid for your street bike. Race spec brake fluid is generally more resistant to heat, but has a much shorter lifespan before it degrades and stops performing. Fine for race teams who change fluid daily, pretty stupid for a road bike. Be realistic and use the recommended fluid – you are not Marc Marquez. 

Bleeding your brakes – what could go wrong?
Many brake systems on motorcycles nowadays incorporate Anti Lock Braking – that’s ABS. It’s simply not worth messing around with these sensitive, computer-driven systems. A wrong move can see you trailering your bike off to a specialist technician to set it right after you stuff it up.

As for conventional systems, it’s worth knowing what the process is, so let’s take a very broad look at how it works and how you could change your brake fluid should you choose to. But first, a word of warning:
Brake fluid is very corrosive. It eats paint and metal coatings. Fast. Be very careful of spills. Have lots of rags as well as a water hose handy. When you finish up, even if you don’t think you’ve spilled any, it’s good practice to wash down what you’ve been working on with clean water, just in case. 

Time to bleed those brakes!

Let’s get started
A hydraulic braking system consists of a master cylinder – generally incorporated into the lever perch – comprising a cylinder and a piston that is actuated by the brake lever. Squeezing the lever pushes fluid down the line against the backs of the pistons in the brake caliper, which act on the pads. Theoretically, brake fluid can’t be compressed, so the leverage you apply is amplified and becomes braking force where the pads meet the disc, where friction then generates heat.

So, if the point is to replace your old fluid and keep your brakes working at their best, how do you remove the existing fluid from a ‘sealed’ system? Brake calipers feature bleed nipples. In order to ‘bleed’ out the old fluid, you must simultaneously open up the bleed nipple (it has a small hex on it like a normal fastener, often 8mm) as you squeeze the brake lever. Then before you release the lever, you tighten up the nipple again so that air is not sucked back into the system. It’s easier if you are an octopus, or have a helper to do the lever squeezing or manipulate the bleed nipple.

Of course, while this pumping out of old fluid is going on, the level of fluid in your brake reservoir is dropping. So the person in charge of squeezing the lever must also watch the reservoir fluid level, topping up the reservoir with fresh fluid as needed. Because if the level of fluid in the reservoir drops too far, you’ll suck air into the system and be back to square one. There are usually ‘low’ and ‘full’ marks on the reservoir. Don’t overfill it or fluid will overflow when you replace the reservoir cap. Remember that just like oil levels, you must measure these levels with the bike stood upright on level ground and the handlebars even, not turned to one side or the other. More reasons why it’s great to have a helper on hand. 

While veteran home mechanics can manage to release the nipple, squeeze the lever, close the nipple and release the lever while also keeping an eye on the fluid level in the reservoir with no assistance, it is different for newbies. The first time you attempt this process, you will stuff up the release-squeeze-close-release sequence, spill brake fluid all over the floor, bump the handlebars and spill what’s in the reservoir as well, trip, swear and wind up with brake lines full of bubbles. It will very likely cost you an extra container of fresh brake fluid before you’ve nailed the process and removed all the air bubbles from your braking system. It can be frustrating, messy and tedious, but it’s very satisfying when you’ve finally sorted it all out. 

Remember that when you bleed your brakes, you are disturbing the reservoir cap seals as well as the bleed nipples. Check these carefully when you finish the job for seepage or even leaks. Everything must be properly fastened and dry. If you are new to working on your brakes, make a note to check everything again in two or three day’s time. As much as we don’t like to nag, you can’t be too careful.

The basics of it are simple. Perhaps what you really need to ask yourself is whether you are patient enough to keep at it until you’ve defeated every last little air bubble, and have a braking system full of fresh fluid with a brake lever that feels rock solid? There’s potentially plenty of mess and frustration along the way, and only you know whether you are the sort of person who can persist until it’s perfect. Because nothing short of perfect is safe.  

Summing up
If you are willing to take your time, do your research and exercise real patience, brake bleeding might be something you could do yourself. But you need to get it 100 percent right. Do you have alternative transport? You might need it if you’re not certain you’ve done the job right.

For most riders, fresh brake fluid is best incorporated as part of a regular servicing schedule by expert technicians like those at Bikebiz. There’s a lot to be said for peace of mind, especially seeing as brake fluid is just one of the fluids included in a service schedule that generally covers oil and coolant, too. 

Besides, don’t you have something better to do on a Saturday than cover yourself in brake fluid? 

We hope you’ve learned something about your brake system through this blog. It feels good to not simply ride a bike, but understand a bit about how it works. And remember, if you have any questions about your brakes, or anything at all about how your bike works, there’s always a Bikebiz expert ready to talk with you about it – just give us a call.

Until next time, enjoy the ride!

This advice is a guide only. It is general in nature. It cannot be relied upon for you to make decisions. All efforts are made for information to be accurate at the time of publishing. If you are unsure of your skills, please take your motorcycle to a qualified motorcycle workshop, and the mechanics can do everything for you.

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