If you read our recent Battle Of The 125cc Ankle Biters, Part 1, which included the Kymco K-Pipe, you know that it had some front brake issues that “Guns” Roderick decided to use as an opportunity to work on his arm strength via repeated hand motions – namely bleeding the bike’s front brake hydraulics. While the brake’s power was improved, it never reached the performance level we felt it was capable of achieving. Naturally, the topic of brake bleeding technique came up. This oft-neglected maintenance task is fairly easy and should be done to replace the hydraulic fluid at the intervals recommended by your bike’s manufacturer.
Aside from bubbles in the brake system, another more common factor can lead to poor brake performance. Most hydraulic fluids are hygroscopic, which means they have a taste for water and will gradually suck moisture past the rubber seals in your calipers. If your fluid is contaminated with water, heavy brake use will raise the temperature to the point where the water will boil (at a significantly lower level than pure fluid would), leading to brake fade and the dread-inducing, lever-to-the-grip braking experience. Also, if you use your brakes really hard, the heat can cook the fluid thickening it.
Street riders are advised to swap out hydraulic fluid every year or two. Some people claim you should do this as part of the winterizing process, so that the moisture-free fluid sits in your bike all winter. Having never had problems develop while my bike was in storage, I prefer to flush the fluid at the beginning of the riding season so that I get the benefit of fresh fluid in the spring. Those whose priority is maximum braking performance will, most likely, want to follow this schedule, too. However, if you suspect that the system has water in it, you won’t want to let it sit over the winter, now would you?
Only basic tools are required for this task, but you can buy some specialized tools if you like. The Mityvac bleeding system is perfect for bone-dry systems – such as when you’ve installed stainless steel lines. If the system is already primed, an old jar and clear hose will work just fine. The fluid you add to your system should come from an unopened container. Remember hydraulic fluid’s thirst for water. (Although DOT 5 fluids are silicon-based and, therefore, don’t absorb moisture, they should be avoided unless specifically recommended by your bike’s manufacturer.) Most OEMs recommend either DOT 4 or DOT 5.1 fluid. Don’t mix DOT 4 and 5.1. Also, make sure you buy a name-brand fluid.
Buy more fluid than you think you need. (If you’re having trouble with bubbles in your lines, the best cure is to run lots of fluid through the system to sweep them out.) Finally, some OEM owners manuals say not to mix brands of brake fluids. If you need to top off the reservoir and don’t know what type or brand of fluid is already in the system, flush the entire system to avoid any problems of interaction between the different formulations, even though – in theory – all fluids should play well together as long as you don’t mix DOT 4 and DOT 5.1. While I’ve added a different brand when I needed to top off the fluid on a tour, I generally bleed the whole system when switching brands.
Begin with your bike on a stand. Since brake fluid damages paint and some powder coats, remove any vulnerable bodywork or cover it with rags. A preparatory step required for those using vacuum bleeders – and optional (but recommended) for all others – is to wrap the threads of the calipers’ bleeder valves with teflon tape. Vacuum bleeders create so much suction that they may be able to draw air into the system past the bleeder valves’ threads, making it impossible to tell when all air has been removed from the system. Teflon tape fills the minuscule gaps between the threads when the valve is not completely closed and is a worthwhile step for all calipers.
One warning, though: Don’t just remove the valve with the caliper mounted to the bike. Since the master cylinder is higher than the caliper, fluid will leak out all over the place. Raise the caliper equal to or above the reservoir to keep the mess to a minimum. Of course, if your lines are empty – as when you’ve just added stainless steel ones – this is not a problem.
Rear calipers have their own idiosyncrasies to consider. Since the brake line travels horizontally from the master cylinder to the caliper, air bubbles can get trapped in the line more easily. To assist the bubbles in their travels, unmount the caliper and hold it higher than the master cylinder, allowing the bubbles’ tendency to rise to keep them moving as you run the new fluid through the system. If you’re pumping the pedal, make sure you place a spacer between the brake pads to keep the piston from popping out of the caliper.
When using a vacuum bleeder, begin the bleeding by sucking the excess fluid out of the master cylinder. Then wipe any visible grit out of the reservoir. If you’re not using a vacuum bleeder, don’t worry about running the old fluid through the system — unless you see dirt or other visible impurities. Don’t add fresh fluid to the reservoir until most of the old fluid has been pumped out of it into the system itself.
Swapping the fluid in the system is pretty easy. With dual front discs, start with the caliper which has the longest total hose length from the master cylinder. Put a box-end wrench on the caliper’s bleeder valve, then press a length of clear hose over the nipple. This must be a snug fit. The other end should empty into a container that is capable of standing on its own. (Glass containers, while breakable, are usually heavy enough to stay put unlike plastic bottles or aluminum cans.) Slipping the hose through a hole in the screw-on cap is ideal. Don’t forget to leave a breather hole, or the built-up pressure could pump the fluid out of the hose when disconnected from the nipple, creating another mess.
Pump the lever a few times and hold the last squeeze. Using the wrench, open the bleed valve until the pressure forces fluid into the hose. Hold the lever until you’ve closed the valve. Repeat this process several times until you have a couple inches of fluid in the hose directly above the bleeder. Now, open the valve slightly – only until you can just squeeze fluid out with the lever and no more. Continue to squeeze and release the lever until you see the fresh – usually clearer – fluid emerge from the bleeder valve.
Pay special attention to the reservoir while you are pumping the fluid through the system. If you let it run low, you’ll get air in the system and have to start the bleeding process from scratch. If you started with empty lines, plan on running several more ounces of fluid through the system after you stop seeing bubbles to make sure that none remain.
When you’re sure that the hydraulic system is free of old fluid and/or bubbles, slowly squeeze the lever as you tighten the bleeder valve. Pump the lever several times and hold. Open the bleed valve to release the fluid. Do this several times. You’d be surprised how often one last bubble pops out during these final, high-pressure bleeds. If one does, run a few more ounces of fluid through the system. You will be rewarded with firmer lever feel.
Torque the bleed valve and repeat the process with the other caliper if there is one. Top off the fluid in the reservoir. Make sure that the rubber diaphragm on the master cylinder is clean before placing it on top of the hydraulic fluid and tightening down the reservoir cover. Wipe away any traces of the brake fluid before it has a chance to damage any painted surfaces. Finally, as with all waste fluids from your bike, be a good citizen and recycle the old stuff.
Those of you with vacuum bleeders shouldn’t feel left out. All of the steps are the same, except that the fluid will be sucked from the bottom end of the system instead of forced from the master cylinder. The same cautions apply, though. If you accidentally let the reservoir go dry, you’ll need to start over.
Any discussion of hydraulic system bleeding usually includes a debate about which method is better: the master cylinder push-through or the vacuum tool suck-through. While many mechanics will subscribe to either one or the other, I’ve found a combination of the two works best. When bleeding dry lines, nothing gets them primed quicker than a vacuum bleeder. However, when swapping fluid, I prefer the pump-through method. So, when installing new lines, I use both methods. First, draw the fluid through with a suction tool, followed by the final flushing of air with the master cylinder method. Regardless of which technique you incorporate, the goal is to fill the system completely with fresh hydraulic fluid and no air bubbles. You’ll be glad you took the time to do this the next time you grab a handful of brake.