If you’ve got juice brakes on your rig, let me blow your world apart for a moment.
Blaise Pascal, a 17th-century mathematician and scientist, came up with an idea that later became a scientific law. Basically, he postulated that when pressure is applied to a fluid, the fluid transmits that pressure equally in all directions. And thus, all the motorcyclists in the world have one square math nerd to thank for the gift of stoppies and skid-stops.
I'll explain this in practical terms. If you have hydraulic disc brakes, that 17th-century technological breakthrough explains why you stop. When you squeeze your lever or stomp your pedal, a linkage connected to your master cylinder’s piston exerts force on the juice within — brake fluid! The pressure expands equally in all directions, and the most movable item moves. That item would be the caliper piston. That expansion shoves the pads up against the rotors, dragging you to a halt.
“Bleeding” your brakes becomes important because air, a gas, is compressible. If there’s air in the lines, it compresses, and the lever or pedal moves, but the piston doesn’t budge. This manifests itself as “soft” or “squishy” feeling brakes — if you’ve got any braking power at all! Bleeding is the process of chasing the air out and sealing the system once again.
The process is fairly straightforward on a bike that has had fluid in it: Pump up the brake to achieve pressure, open the bleeder without releasing the brake to expel air, close the bleeder, allow the pedal/lever to return to its resting position. Repeat until the brakes have good feel.
Here’s the problem: folks get nervous about it. And they should. It’s brakes. Inoperable brakes are a Very Bad Thing. So let me help by spelling out some of the things I took years to learn or spaz my way into. The following tidbits should supplement the video.
Cover up, v1.0: Unless you need new paint. Brake fluid kills paint. If you value your bike’s shiny bits, cover them.
Cover up, v2.0: Put the top back onto the master reservoir. You don’t have to screw it down tight, but give it a turn or two. As you start to build pressure by pumping, geysers can — and do — form. Splashing onto the top and gasket is no problem. Splashing fluid onto the paint (if you ignored Cover up v1.0) is a problem.
Start bleeding at the farthest port from the master cylinder: Your manual may have a specific order to do things, but if it does not, start at the farthest item from the master cylinder. For dual-disc front brakes, that usually means the left front caliper. You can technically do this in whatever order you please, but if you have lots of air in your lines (like if you just put on some new braided stainless steel brake lines), this will drive the most air out quickly, leading you to pump less. Move over to the next caliper, and to the bleed port at the master cylinder if the bike is equipped.
Remember, brakes are a sealed system: If you are losing brake fluid regularly, something is wrong and needs more attention than a simple bleeding job. Similarly, since the fluid level drops as the pads wear, don't just add fluid helter-skelter. If you do, at your next brake job, you run the risk of pushing fluid out of the reservoir when you drive the caliper pistons back into their bores.
Keep the master full: Forget this rule, and air will get sucked into the fluid as you pump the brakes. You will then start all over again.
Use the right fluid: There are a lot of flavors of fluid out there. Some are based off of castor oil, some off of glycol, and some are silicone-derived. None of those bases play well together. Be sure you’re putting fluid with the correct formulation and boiling point in your bike. Some mix, some do not. For instance, DOT 4 can replace DOT 3, but not the other way around. DOT 5 and DOT 5.1 sound similar, but those cannot be mixed. If you don't know what is safe to mix, then don't do it. The people who engineered your brake system are smarter than you, I promise.
The easy way to swap fluid is to do an extended bleed: Bleed out as much fluid from the master as you can without going dry, then refill it and do it again once or twice. It’s usually easier because the master never goes dry, so you don’t have to worry about what to do if you can’t get any pressure at all when you’re starting.
Gravity bleed when you can’t get pressure: Fill the master, crack the bleeders, and wait for the fluid to show up at the bleeders. Gravity will help draw fluid through the system, sometimes getting you enough pressure to finish the job normally. Keep in mind this only works if the caliper is lower than the master cylinder.
Bottoms up!: If your bleeder is not facing upwards, you might never “get a lever.” Air bubbles travel upwards, and liquid travels downward. If your air bleedin’ fitting points down, you’ll have trouble forcing air to go the “wrong” way. Just loosen the caliper and rotate it until the bleed port faces upwards. Remember to leave the pads in contact with the rotor, or you’ll have to drive them back in!
If you’re a rookie at brakes, watch the video for some more thought and theory on brakes and a walk through of what you’re facing. That should make these tips easier to understand and implement. I’ve been exposed to lots of methods over the years, and these are the ones that have flat-out worked.
If you need some help, drop me a comment down below and I’ll see if I can’t help you out some, and I wouldn't be surprised if some of our wrenchy readers provide some tips, too!