How to bleed motorcycle brakes and fit new braided lines

Keeping your motorcycle brakes working at their best is vital, but because they feel okay for every-day riding, they’re often neglected. Here we’ll show you how to change your bike’s brake fluid, as well as how to fit a new pair of braided brake lines…

How do I look after my brake fluid and lines?

Just like everything on your bike, you should keep your brake lines clean, but otherwise they’re relatively maintenance-free. The brake fluid reservoir should always be kept closed, and make sure there are no leaks from it.

How long should my brake fluid last?

Your brake fluid needs to be replaced every two years – over time it absorbs water, which means that under hard braking the tiny pockets of water caught inside will boil, forming gas that causes the brakes to fade and fail. Under general use you’ll likely not feel much difference, but it’s under heavy braking, perhaps in an emergency, that old brake fluid will let you down. When it’s gone brown, it’s really had it; if you’re looking at buying a second-hand bike with such clearly old fluid, question how well the bike’s been looked after.

DOT 4 brake fluid is most commonly used in motorcycle brake systems – when new it’ll boil at over 260°C. It’s considered at the end of its life by the time it’s absorbed 3.7% of its volume with water, at which point it’ll boil at 180°C. In normal use that’ll happen in about two years, but if you left the lid off your brake reservoir, it’d happen within a day or so; that’s why you must always use brake fluid from a new, sealed container.

To find out more about brake fluid, click here.

How long should my brake lines last?

Motorcycle brake lines are typically made of rubber, with a braid inside to stop them swelling. When new, they should perform perfectly, but over time they’ll start to give, resulting in a less direct feel at the lever. They’re also considered a replaceable part; my 1999 Kawasaki ZX-6R’s service schedule states that the brake hoses should be replaced after four years.

To replace the three rubber lines that make up the OE system on the front of the ZX-6R would cost over £180, while replacing the rear would be almost £60. But a set of braided lines will be much cheaper, and last longer. I fitted a set of Venhill hoses for just £56.47 on the front and £28.99 on the rear, which included new stainless steel banjos and all the copper washers required.

Quality braided lines with a Teflon liner like these should last a lifetime. To find out more about brake lines, click here.

My bike already has braided brake lines – should I change them?

There’s no need, unless they’re damaged. That will usually be visible, but the other risk is that cheap ones were fitted with nylon liners that can melt in extreme circumstances. On a bike, that’s unlikely to happen through heat generated by braking, but it can occur if the line presses on the exhaust manifold, leading to brake failure. Don’t buy cheap brake lines.

Only ever fit quality, tested brake lines

Can I upgrade the brake lines on a bike with ABS?

Usually, yes you can. Some early ABS bikes had the hoses crimped into the pump, but they now tend to be fixed with banjo bolts. Brake lines are generally priced by the line, not by length as the majority of the cost is in building them; as bikes with ABS have so many more lines, expect to pay extra – Venhill hoses for a 2013 ABS-equipped bike, for instance, cost £129.73 for the front and £82.42 for the rear.

It’s also vitally important that the lines are not muddled up, something that could be surprisingly easy given the potential number of connections; get it wrong and it’s possible for the brakes to completely lock up in an emergency, as happened in at least one recorded incident. If in any doubt, speak to your dealer.

The difficulty with fitting new lines to ABS bikes can be in ensuring the fluid is fully replaced…

Inside an ABS pump

While this is a car ABS pump, there’s only one more caliper involved; it’s a fascinating insight into the workings of ABS, and it’s fairly clear why you need to be careful when working on modern braking systems.

Can I bleed the fluid on a bike with ABS?

Yes – ABS bikes use brake fluid just like pre-ABS machines. Modern fluids have been refined to work at their best in all temperatures with ABS (it’s even more important that you use the rating of fluid specified), but some manufacturers say that their ABS pump systems have to be primed after a fluid change, using their workshop diagnostics equipment.

Most bikes use various braking systems from the likes of Bosch and Continental, and a head-office mechanic from one of the world’s biggest motorcycle manufacturers told me that normal bleeding should be fine for the average home mechanic, as long as NO air enters the system. That means that changing brake lines could be problematic. The best recommendation would be to speak to your dealer; never work on assumptions with braking systems.

What parts do I need?

In this article we’ll show you how to replace your motorbike’s brake lines, and then fully bleed them, but the bleeding process is the same if you’re just carrying out a brake fluid change.

If you’re replacing the lines, buying a set from one of the UK manufacturers like Venhill will mean you have everything you need besides the brake fluid.

When you order your lines, you’ll be able to specify what layout you want them – to understand the difference, click here. I went for a two-line layout (often called ‘race layout’ but not because it’s any better than the others). If your bike has been modified, for instance with bar risers, you might need slightly longer lines, but they can be made to order (Venhill has a 48 hour turnaround) once you’ve measured them. Your lines mustn’t get snagged up anywhere and they must be long enough that they’re not pulled tight when the suspension is at full extension. If you’re in any doubt, contact the company that’s making the lines.

Good quality brake line kits will come with all the parts you need – I opted for stainless steel banjos, but other materials – including titanium – are available

Whether you’re replacing the lines or not, you’ll need brake fluid. You MUST use the brake fluid that’s specified for your bike – check the specifications and also the top of the reservoir cap (pay extra attention if your bike has had an aftermarket master cylinder or calipers fitted). Note that silicone-based DOT 5 brake fluid is NOT compatible with the other glycol-based fluids like DOT 4 or DOT 5.1. The choice of fluids is explained here, but we strongly recommend that you only use what is specified (usually DOT 4 for modern bikes).

You MUST only use fresh brake fluid from a new, sealed container. As I was fully replacing the front and rear brake lines, I used a 1 litre tin of Motorex DOT 4 fluid; you wouldn’t usually need this much for a bleed, but it worked out better value than buying smaller bottles. Remember that you can’t store any unused fluid once the bottle’s been opened, and it must be disposed of properly at your local recycling centre.

Note that brake fluid is extremely damaging to paint, so be very careful not to spill any, and be ready to wash any spills immediately.

What tools do I need?

This article isn’t intended to be all you need to change the brake fluid or lines on any motorcycle. I’m basing it on my 1999 Kawasaki ZX-6R, but your bike will have its own unique needs, so I thoroughly recommend buying a Haynes workshop manual. My hope is that this article will give you the confidence to take on the job for yourself, but only with a workshop guide specific to your machine will you have the correct procedure. Remember that Bennetts customers can save a massive 40% on Haynes manuals at Bennetts Rewards. You’ll also need the following…

• Socket set

• Spanner

• Hex-head sockets

• A length of clear tube to fit the bleed nipple

• A container for the old fluid – I use an old coffee jar with a hole in the lid.

• Torque wrench (optional but recommended)

• Screwdriver (if required for reservoir cover)

• Protective gloves

How do I change the brake lines and bleed the brakes on my motorcycle?

We’re fitting a full set of lines here, but that process is fairly simple; it’s the brake bleeding that’s more complex. We’ll be working on the front brakes here, but the process is the same for the rear. Please do ensure you use this as a guide – I’m working on a simple older bike, but you should follow the process in your Haynes manual for you own machine, especially if it has ABS or linked brakes…

Step 1: Protect your bike

Brake fluid will ruin your bike’s paintwork so wrap some cloth around the bottom of the reservoir and anywhere else that fluid could spill. Have some water and towels ready, just in case it all goes wrong. Clean around the reservoir cap, all the banjos and the bleed nipples, then set the bars at such a position that the reservoir is as level as possible (make sure you don’t knock the wheel while the reservoir is open). If you’re not changing the brake lines, skip to step 6.

Don’t worry – you don’t need to own a vacuum pump to do any of this work.

Step 2: Empty the fluid (only if changing lines)

This is where a vacuum pump can come in handy; I have one that I bought when Hein Gericke still existed, but Venhill does sell one. Remove the reservoir cap and rubber diaphragm, then lift off the bleed nipple dust caps before popping a spanner over the nipple on the caliper – I put a ring spanner over it so it’ll stay there, leaving room to open and close the nipple. You can then connect the vacuum pump, build up the pressure, open the bleed nipple a crack then draw all the fluid out of the system (on both sides if you have two front calipers)

If you don’t have a pump, pop a length of tube onto the bleed nipple instead, then put the other end into a container – I have a hole cut in an old coffee container that the hose passes into. Crack the bleed nipple open then pump as much of the fluid out of the system as you can. Repeat on the other side if you have two calipers up front.

Step 3: Remove the brake lines (only if changing lines)

Before taking them off, look carefully at how the lines are fitted – check the angles of the banjo bolts at each position and take note of how the lines route around the steering; takes some photos if you need to. If you didn’t use a vacuum pump, there’ll likely be some fluid left in the system (there probably will be even if you did) – unscrew the banjo bolts and carefully remove them with the lines and the copper washers. Be really careful not to spill any fluid on your bike.

Step 4: Fit the new brake lines (only if changing lines)

Using the new banjo fittings and copper washers that came with your brake line kit, pop the banjo through the end of the line with a washer either side. If two lines go through one bolt, put a washer between the two lines as well.

The ends of the lines might be angled, so check carefully which way they should go. Get the lines set up but don’t tighten them right down yet – thoroughly check the steering and full suspension travel to make sure the lines can’t get trapped or foul anything. Only once you’re sure should you tighten the banjos down to the specified torque (on this bike, it’s 25Nm).

Before tightening, do make sure the lines have passed through anything they need to – for instance there’s a plastic guide on the swingarm that the rear brake hose has to be passed through before it’s fixed to the master cylinder and the caliper.

Venhill’s lines have rotating end fittings, which make getting the lines perfectly set up a lot easier – as they’ll have been loose when you fitted the lines and nipped up the banjos, they should be sitting perfectly; all you need to do it tighten then up with a spanner.

On the left is the fluid that came out of my bike; it’s more than nine years old and way beyond its best. Only ever use quality brake fluid from a brand-new container. I’ve got a litre of Motorex, though for general bleeding you’ll probably only need half a litre at most.

Step 5: Fill the system with fluid (only if changing lines)

With your tube over the banjo (nip the other one up if you’re working on a twin front-brake set-up), slowly and carefully fill the reservoir with your brand new brake fluid while pumping the brake lever. Once fluid is flowing through the pipe into your container you can nip up the bleed nipple. If you have twin front brakes, move onto the other caliper and repeat, keeping the reservoir topped up as you pump the fluid through; I found I had to rapidly pump the lever to get the fluid moving.

If you have a vacuum pump you can more easily pull the initial fluid through the system to each banjo.

Once you’ve got the fluid through both lines, make sure the bleed nipples are closed (don’t overtighten them), then you MUST bleed the system…

Look for air bubbles coming up when bleeding the master cylinder

Step 6: Pump the lever

Changing your bike’s brake fluid and bleeding the system requires moving all of the fluid through the master cylinder, hoses and calipers, replacing it with fresh. There may be air in the lines, but once you’ve moved it all out and replaced with fresh, it should be clear.

Pop the reservoir cap off if you haven’t already, then slowly pump the brake lever – you might see some air bubbles appear; keep pumping until you don’t see any more to complete bleeding of the master cylinder. You can now loosely refit the reservoir cap.

Step 7: Attach the bleeding kit

Lift off the bleed nipple dust cap (if it’s missing or damaged, make sure you replace it), then pop a ring spanner over the nipple, leaving yourself space to open and close it. Then fit your clear pipe, the other end going into the container, which should have enough brake fluid in it to submerge the end of the pipe.

It’s important to never let the reservoir run dry when bleeding the brakes, and you must pour fluid into the reservoir slowly, so as not to introduce any air bubbles. At all stages, keep an eye on the fluid level – if it drops below the bottom of the reservoir, you’ll have to start all over again.

Step 8: Bleed all of the air out

Slowly pump the brake lever four times then hold it in while cracking open the bleed nipple – you’ll see fluid flow into the pipe and the lever will go light. Continue pulling it all the way in (or pushing it down if it’s the back brake), then close the bleed nipple when you get to the end of the stroke.

Now slowly release the brake lever.
Keep doing the same process until there’s no sign of any air coming out of the bleed nipple, then move over the other caliper if it’s a twin system and carry out the same process.

If you’ve fitted new brake lines, it’s worth doing both sides twice, to make sure both lines are fully bled. You might also struggle to get all of the air out – I did – but persevere with this method and it should clear.

If the lever is pulling all the way back with the bleed nipple closed, you have a lot of air in the system or some is trapped; this happened to me after changing the lines on the ZX-6R, and was worst at the back. By pushing the brake pedal down about a third of the way (it’s the same principal with the front brake lever), then cracking the bleed nipple and continuing the press down – closing the nipple at the end of the stroke – I was eventually able to get pressure building in the system so that I could bleed it properly. It can help to remove the caliper and hold it up in the air, with the bleed nipple at the top (so the air is more inclined to rise) if you’re having real problems; it’ll probably be worth having someone help you.

If you have a vacuum pump, you can use it to draw the fluid through the system while keeping the master cylinder topped up, but honestly, I don’t use mine for bleeding – I never seem able to do as good a job as I can by hand.

Expert tip: Beating a difficult system

“If you’re struggling to get the fluid though the system due to an air lock, try cracking the banjos open at the calipers with the lever under pressure,” says Nick Nomikos from The Two Wheel Centre in Harpenden. “Tighten them back up, then do the same at the bleed nipples, then any junctions, then the banjo on the master cylinder. Of course, if the master cylinder has a bleed nipple, use that.

“This could be messy, so be very careful if you do it.”

Step 9: Clean up and finish

Clean the diaphragm under the reservoir cap, and the cap itself (the diaphragm should separate), then pop it back on. Clean up any spilt fluid and double-check that everything is nipped up tight then make sure there are no leaks.

The brake lever should feel firm to pull, and won’t creep back to the bar (or drop all the way down on the back brake) even if you keep it pulled hard.

If you’re in any doubt at all, check all your work, and if you can’t get it right, or you’re in any way unsure, have it checked by a professional mechanic. When you do test ride your bike, take it easy while you make sure it’s working correctly.

If you can’t do this job yourself, your dealer will be happy to take the work on. While prices will vary, Nick Nomikos, owner and MoT tester at The Two Wheel Centre in Harpenden, says he’d charge £66 inc VAT to replace all the brake fluid on a bike. To change the hoses as well would add £50. These prices don’t include the fluid or the hoses.