At the Super D downhill mountain bike race in Ashland, Ore., in June 2010, I got to rub shoulders with some of the techies from SRAM, the race sponsor, who’d flown in from Chicago. I overheard one of their engineers talking about brake bleeding, the act of removing performance-robbing air from your hydraulic braking system. He was only too happy to fill me in on the topic over a beer later that day. As it turns out, brake bleeding should be performed once a year, and is straightforward enough to do yourself (keep in mind that “straightforward” to engineer-types may or may not be straightforward to all of us). If you ride frequently or in aggressive terrain, it's best to bleed your brakes about every four months.

How do you know your brakes need to be bled? They start to feel mushy. Instead of taking your bike to the shop next time, read on to see if brake bleeding might become your next bike maintenance skill.

The Basics
Air enters all hydraulic braking systems over time, getting trapped in the hose, caliper, or lever. Your job is to chase those bubbles out before the amount of air begins to degrade your braking performance. Before you start, determine if your brakes use mineral oil or DOT Fluid. Most use mineral oil, but brands like Avid Juicy use DOT. It’s important that you use the correct oil and avoid spilling any oil on your brake pads. A pad contaminated with oil will need to be replaced.

Most brake manufactures sell bleed kits for about $40 that come with the proper oil for their braking system, along with syringes fitted with hose clamps, a bleed block, and a wrench appropriate for the screws in their system. You’ll need to supply safely goggles, rubber gloves, a towel, a small screwdriver or pick, and a hex wrench.

Step-by-step instructions will vary by brand, and can be found in the technical documentation for the braking systems. A local bike shop that deals your brand of brakes will have them, or even better, some manufacturers make them accessible online. For example, SRAM’s brake bleed details are found on page 60 of their brakes manual.

Nitty-gritty details aside, here’s an overview of the blake bleed process, which will apply to any brake type and give you a sense of what you’re getting yourself into.

Process Overview
(Note that you do one side at a time.)

1. Fill one syringe ½ full of oil, and the other ¼ full. De-gas them by closing their bleed clamps and pulling the syringe to suck out trapped air you couldn’t even see before.
2. Tie your brake lever to the bar or squeeze and hold. The tie-off is better since it leaves two hands free.
3. Bleed the brake caliper by sucking any air out into the ½ full syringe.
4. Untie the lever.
5. Push the air-free fluid up through the hose, through the lever, and into the other syringe (the ¼ full one) attached to the lever.
6. While holding the lever, push the fluid through the system until the lever returns to its position.
7. Bleed the lever by sucking any air into the syringe. Close the bleed ports.
8. Empty the syringes into a sealed container and dispose of the fluid properly. (Used fluid should be recycled or disposed of in accordance to local and federal regulations.)

Before you ride, test your brakes. Pull on the lever several times, as hard as you imagine you’d pull it while out riding. Check for leaks around the hose nut on the lever and the bolt on the caliper.

Sound like fun? At the very least, it will keep your brakes functioning at their finest and impress the heck out of your gear head friends.

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