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Brake Bleeding Basics

by Sportbike Solutions

Brakes that work, and work well, are an absolute necessity on motorcycles designed to exceed most interstate speed limits in first gear alone. Still, hardly a day goes by that we don’t witness the results of brake system neglect, ranging from mushy levers and brake fade all the way up to brake failure and lockup. Sadly, most of these problems could have been easily averted for the price of a bottle or two of brake fluid and a few minutes time every few months.

As a general rule, performance motorcycles are designed to use DOT 3, 4, or 5.1 fluids … all glycol based. The performance advantages of glycol-based fluids make them the only real answer in hydraulic systems where consistent lever feel and action are mandatory. However, glycol-based fluids come with a price, as they are easily degraded by exposure to atmospheric moisture and ultraviolet radiation (light). While there are steps that can be taken to minimize these factors, the only way to reliably keep your hydraulic brake system in tip-top shape is to routinely flush, bleed, and replace your brake fluid.

How often is often enough? For street-driven motorcycles, we recommend changing brake fluid and bleeding the system at least every 6 months. On race bikes, we recommend doing so before each event!

In our experience, the biggest stumbling block to routine brake system maintenance is the process of bleeding air from the system. On the surface, this would appear to be a relatively simple procedure, but as anyone who's had a 'problem bike' will attest … it can be far from simple at times! This is especially true when starting with a dry brake system, as when changing lines.

We won't attempt to rehash the basic procedures of bleeding. There simply isn’t a lot to say on the subject that hasn’t been already been said. But there are a few tricks of the trade that we’ve found to be of benefit:
  1. ALWAYS start with a freshly opened bottle of fluid. Glycol fluids are extremely hydroscopic, which means they take on moisture from the atmosphere. Absorbed moisture lowers a brake fluid's boiling point, which results in brake fade. Even if you’ve closed a bottle tightly, the air trapped within can contain enough moisture to contaminate the contents. It's best to purchase small bottle of fluid and open one each time you service your brakes, rather than keep one large one on the shelf.
  2. Use a vacuum pump. The Miti-Vac is one example, but there are plenty of other similar pumps on the market that work equally well. By using a vacuum pump to draw fluid from the bleeder screws, you can expedite the process and all-but-guarantee you won’t accidentally re-introduce air into the system. If you're serious about brake bleeding, a vacuum pump is a valuable commodity you will not regret purchasing!
  3. Tap, Tap, Tap. Using a screwdriver or wrench to quickly & lightly tap brake system components, such as the lines, calipers, and master cylinder, can help move small air bubbles up and out of the system. Next time you're drinking a carbonated beverage, examine the gas bubbles clinging to the edges of your glass. Air in your brake system does the same thing. Tap your glass. See the bubbles rise? The same thing happens in your brake system. Tap, tap, tap.
  4. Think like a bubble. It may sound silly, but keeping in mind the simple fact that air bubbles rise can save you a lot of time bleeding brakes. Particularly in OEM arrangements, there are often brake line junctions where air can be trapped. Sometimes, in order to get a good bleed, you'll need to temporarily remove and re-align components in order to give air a straight shot to the top of the system.
  5. Work up from the bottom. Start by bleeding the calipers. Then bleed at the master cylinder. Air rises. You should also rise up as you work to bleed the system.
  6. Collapsing pistons. In some systems, air gets trapped behind the caliper pistons, which can lead to serious frustration! It often helps to remove the calipers, one at a time, and push the pistons all the way back into the caliper body. Make sure to keep an eye on the level of fluid in your reservoir as you do this, as it can overflow if you're not careful. When doing this, we usually siphon off most of the fluid in the reservoir first, and then refill to the top before pumping the brakes.
  7. Bleed at the master cylinder. Since air rises, this is usually the final and most effective step to bleeding the system. Most of the newer radial master cylinders are now incorporating bleed screws, which make this step a breeze. But if you don't have such luxuries, you can either lightly crack the banjo bolt at the master cylinder while maintaining lever pressure (keep a rag underneath to catch fluid), or purchase a banjo bolt with a bleeder screw incorporated. The latter is definitely our option of choice.
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