Transcript
Technical
Back-Bleeding Hydraulics by Mark Barnes
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pedals rob hydraulic brake and clutch systems of precise operation, reducing rider confidence. We covered some bleeding methods in our recent brake maintenance article (April 2013), but did not cover back-bleeding. Back-bleeding is a fast and effective way to get stubborn air bubbles out of hydraulic lines, and it’s especially good for refilling a dry system. Traditional bleeding forces fluid from the master cylinder to the slave (either the clutch slave or brake caliper) by simply pumping the lever or pedal, but this can take a long, long time for an empty system, since the master cylinder piston only sips tiny amounts of fluid on each stroke. The use of a vacuum pump to pull fluid through the system and out of the slave bleed valve is much faster. But both these methods can struggle to get those last tiny bubbles out of a high point in the line, since air keeps trying to rise as fluid passes downward. As its name implies, back-bleeding pushes fluid from the slave to the master cylinder. This technique is both faster and better at forcing out air trapped high in the system. All that’s needed is a large syringe (check auto parts stores and medical or veterinary supply shops), some clear vinyl tubing, and possibly a hose clamp (depending on how tightly your tubing fits on the slave bleed nipple). You’ll also need fresh brake/clutch fluid, a wrench to open the bleed valve (usually 8mm), and some rags to catch any spillage. Note that brake and clutch fluid aren’t always the same; some Euro bikes use mineral oil in their clutches instead of brake fluid. Always double-check that you’ve got the correct supplies. Finally, if your bleed fittings don’t seal well—and many don’t—Teflon thread-sealing tape may be required. Make sure to wrap it clockwise (with inboard tip facing you; 2-3 revolutions will do), and trim any excess well away from the tip. You don’t want a piece breaking free and clogging some tiny orifice elsewhere in the system. Unfortunately, you won’t know how well your fittings perform until the bleed process is underway, so have something handy with which to plug the fitting port if you need to remove the fitting to wrap it (e.g. a spare fitting or a soft plug, usually about 7mm in diameter). Snug-sealing bleeder fittings are important for any type of bleeding, not just back-bleeding. First, attach the clear tubing to your syringe; secure with a zip-tie if necessary. Place the tube’s open end deep into your new fluid and withdraw some by pulling the syringe plunger outward. Stop short of the syringe’s capacity by 25%. Now point the tube and syringe upward, holding the line straight. Tap the assembly until all air is at the top of the tube. Squeeze the plunger in until the air is expelled and fluid has reached the tube’s open tip. Quickly slide the open tip over the bleed nipple, letting as little air into the tube as possible during the process. Use a projection on the bike or a piece of wire to suspend your assembly above the slave cylinder for the next phase. 30
DECEMBER 2013 // MOTORCYCLE CONSUMER NEWS
Now open the master cylinder reservoir. Open the bleed valve. You may see an air bubble immediately escape into the clear tube. Tap the tube/syringe assembly until all air is at the top of the syringe, next to the plunger. Squeeze the lever or depress the pedal. If bubbles rise into the clear tube, just keep pumping the lever or pedal until they stop. There’s no need to close the valve in-between pumps, as with traditional bleeding. Make sure you don’t force the plunger all the way out of the syringe during this process. When air stops escaping into the clear tubing, again tap the assembly and get it to the top. Air that was trapped at the slave end should now be gone. Vigorously tap the hydraulic line with a screwdriver handle all the way to the master to free air bubbles trapped along its length; this will help them rise toward A tiny hose clamp secures our the top of the system. Remove clear tubing to the bleeder all the fluid out of the resernipple, as pressure from voir with another syringe or a the syringe might pop the rag (wipe out any gunk while tube off. Note Teflon tape you’re at it). Now push fluid around fitting threads; through the system by depressbleeder fittings can leak ing the syringe plunger, keepwithout it. ing an eye on the reservoir to avoid overflow; remove more fluid if necessary. You may notice bubbles, perhaps quite tiny, emerging from the piston port into the reservoir as it fills with fluid. That air was trapped too far from the slave cylinder to exit easily down there. Be sure not to depress the plunger so far as to force air trapped in the syringe back into the system. If you’re filling an empty system, start by pushing fluid in from the slave up to the master cylinder reservoir. Then finish with a little pumping of the pedal or lever to expel any air that may have been trapped in the slave cylinder, which you will observe in your clear tubing. Continue until no more air bubbles are visible, keeping plenty of fluid in the reservoir. When no more air escapes from either end as you push the plunger or pump the lever or pedal, close the bleed valve, remove your syringe/tube assembly and fill the reservoir to the marked fill line. Replace the reservoir cap and wipe down any spillage at either end with brake cleaner and a fresh rag. It is extremely important to avoid getting brake fluid on your rotors or pads, as it’s a thin oil that can severely compromise brake function; clean those areas very thoroughly with brake disc cleaner. Also, brake fluid can discolor or dissolve paint, so promptly clean any spillage on painted surfaces. Your system should now feel crisp and firm. The process seems complex on paper, but once you’ve got your equipment set up and get some practice, you may conclude that back-bleeding is the easiest and quickest way to fill/bleed a hydraulic system.
PHOTOS MARK BARNES
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