How to Bleed Your Motorcycle Brake Lines with a Vacuum Pump

Air is essential for life. Air is detrimental to your life when in your brake lines. It’s one of the ironies of life. There’s a number of ways to bleed your motorcycle brake lines but by far and away the best method is with the use of a hand vacuum pump. It’s pretty much the way all professionals do the job and with good reason as if you do it correctly you’re guaranteeing that you remove unwanted air in the brake lines.

So why is air such a danger in brake lines? Your braking system is simply hydraulics. The brake fluid (liquid) inside is pushed by your brake lever into the piston, thus pushing the pads onto the brake disk. It’s an incredibly simple system and it works because liquid doesn’t compress under pressure. Air on the other hand, does. So if you have air in your brake lines and you pull your lever in, it compresses and therefore there now won’t be enough volume inside the lines to push the brake piston out and into the pads. That means you won’t have the stopping power you’re after.

At its most basic, if you have a completely empty brake line and you pull the brake lever, you’ll feel no resistance. But even if there’s just a small amount of air, it can be enough to prove fatal.

The difficulty with getting air out of your lines is that while you’re putting new fluid into your lines, the air bubbles are doing their best to move up the line. So while continually flushing your lines you can get the air out of them, it’s not a guaranteed solution. You can end up having to flush them a half or dozen times or more before the brakes feel like they’re air free. That’s why a vacuum pump makes the job so easy because it will suck everything – air and fluid – out of your lines.

Tools you’ll need are:

  • A wrench to open and close your brake bleed valve
  • A brake bleeding kit that includes a hand vacuum pump
  • Brake fluid

There’s many brake bleeding kits on the market and probably the most well known is the Mityvac. But any hand vacuum pump will do. The type we’re using is a specially made one from Bikeservice which comes with all the bits and pieces you need to bleed your brakes – hoses, adapters to go on the bleed valve and importantly, a reservoir for the bled brake fluid to be captured into. It’s also great to diagnose various engine and carburetor issues, too.

The video above shows us doing this with a brake system completely void of fluid. If you’ve got brake fluid in your system but with air trapped inside, skip down below.

Start be opening up your brake reservoir and removing the diaphragm. Pour fluid into the reservoir and begin circulating it through your system. To do this, just push/pull the brake lever in and out gently. Keep doing this – and topping up the reservoir – until you start to feel some resistance. There’ll be plenty of air in your lines still, but putting fluid into them makes the job of sucking both the air and fluid out with the vacuum pump easier.

Next, loosen the bleed valve on the caliper with a wrench and then tighten back up by hand.

Find the correct adapter that will fit over the bleed valve from your kit and then attach clear hose to it. Attach that hose to the kit’s reservoir and finally, attach a further clear hose from the reservoir to your vacuum pump. The ‘circuit’ should look like this:

When attaching the open end of this circuit to the bleed valve, a trick is to position it in such a way that when you open the valve, the hose will hang down and not twist. That means you’ll probably attach it so that the hose is pointing upwards and when the valve is opened, will drop to face down as shown below.

Before opening the bleed valve, pump the hand vacuum about ten times. If everything has been put on correctly, the gauge will show a steady reading.

Now, open your valve and keep pumping the vacuum pump. You’ll want the pump to maintain a reading of between 15 and 20 inches of mercury (a common measurement of vacuum) to ensure that it’s pulling the brake fluid and air out at a steady pace. It can sometimes help to keep one hand on the hose and adapter covering the bleed valve to keep a tight seal and prevent it from coming off.

All while this is happening, keep an eye on the brake fluid reservoir. If it goes empty you’re going to start sucking air into the lines and have start all over again. If you’ve got someone with you, they can continually top the reservoir up, but if you’re by yourself you’ll need to close the valve, top the reservoir up with fluid and repeat the process again.

Depending on how long your lines are and the size of your brake fluid reservoir, that may mean you can only keep the valve open for a few seconds before needing to close it again and top up the fluid.

Now test your brakes – they should offer good feel and resistance. If your bike is lifted off the ground you can check too by rotating the wheel and engaging the brakes. The pads should progressively grip the disc as you put more pressure on the lever and when released, the pads should retract enough for the wheel to rotate again.

Once you’re satisfied all the air is out of the system, tighten the bleed valve up with the wrench and replace the diaphragm and cap on the reservoir.

One thing to keep in mind – don’t let the reservoir that’s connected to your vacuum pump get too high – you don’t want to suck brake fluid into it. For a motorcycle brake system, you’re unlikely to need to suck that much fluid for a single brake line, but more so for a car.

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