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Vacuum-bleeding motorcycle brakes

Martynn's tips motorcycle brake bleeding

I rode my daughter’s motorcycle – a Honda CBF1000 – for the first time a while ago, and was a little alarmed by the front brake lever which seemed a little ‘spongy’, with the lever almost coming back to the handlebar grip under heavy braking.

As far as we could tell, there was no record of the brake fluid having been changed at all, so that seemed like a sensible thing to do first.

How does brake bleeding work on a motorbike?

The CBF1000 has a complex hydraulic braking system. Not only does it have ABS, but it’s also a ‘linked’ system.  When the front brake lever is squeezed, all of the caliper pistons on the left-hand front caliper are operated, along with two of the pistons in the front right-hand caliper, and one at the rear caliper.

When the rear brake pedal is pressed, the remaining rear caliper piston is pressurised, along with the remaining piston in the right-hand front caliper.

The idea is to spread the braking load, which should increase the bike’s stability under braking, but it does mean that there are a lot of hydraulic hoses/pipes, and therefore places where air can get trapped.

Previously when I’ve been working on complex car brake systems, I’ve had great success using a commercially available vacuum system to literally pull the fluid through the hoses and pipes, along with any trapped air.

However, these ‘professional’ systems are expensive, and require a source of compressed air - not an option in this case.

A quick search online revealed that hand-operated vacuum bleeding kits for brakes can be bought for less than $30. They’ve got to be rubbish, surely? Only one way to find out…

Well, the kit arrived in its own plastic case, which contained the vacuum pump, the fluid collector, various hoses and adaptors.

Brake bleeding kit

Motorbike brake bleeding kit

A quick check revealed that the hand-operated pump did in fact work pulling a vacuum which was registered by the integral gauge.

The instructions supplied were pretty poor, but with a little thought it’s pretty obvious how the various parts go together.

So, I started by placing rags around the front fluid reservoir to catch any spilled fluid, then removed the cap. 

Brake fluid reservoir motorycycle

CBF1000 brake bleeding

I removed the old fluid in the reservoir with the pump, and topped it up with new fluid.

The next step was to place a spanner around the first bleed nipple on the front right-hand caliper, and attach the hose from the pump fluid collector.

Here I encountered a slight problem. The kit included 3 different sizes of rubber adaptors designed to fit over the bleed nipple, but they were all too big - probably intended for the larger car-sized bleed nipples. This wasn’t a problem though, as the plastic hose fitted OK without the need for any of the adaptors.

CBF1000 brake bleeding

With everything attached, I operated the pump a few times until the gauge registered a good vacuum, then opened the nipple about 90°, and fluid flowed through, along with a few air bubbles.

If the nipple is opened too far, air will probably be pulled past the nipple threads, and into the vacuum hose. This may not do any harm, but it won’t bleed the system either.

Spanner on brake caliper nipple

After repeating this procedure at the CBF1000’s remaining bleed nipples in the sequence prescribed in the Haynes manual, and topping up the reservoirs, the brake lever operation was much improved. Success!

And the cheap vacuum kit? Well, it had worked fine, nothing had broken, and it is fit to be used again next time. What more could you ask?