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Q: How do I bleed stubborn brakes?

A: Bleeding brakes, depending on the type, can be relatively easy or motivate one to fire up the plasma cutter and wreak havoc on any mechanical components within reach. Here's some quick tips that might prove helpful. First, make sure all components are in good mechanical shape, properly adjusted and tightened. I can't stress this enough. A lot of bleeding 'problems' are a result of poor maintenance. Pressure bleed the system upwards from the caliper. A large syringe and short piece of plastic tubing (about ten inches) that fits tightly on the bleeder screw works best. The best place to find a large, durable syringe is a farm supply store but don't ask what farmers use them for, you don't want to know. Use a six point box end wrench in order to not round off the screw flats.

I like DOT 5 brake fluid (Silicone) and change it twice a year. When topping off the front master cylinder, overfill it slightly in order to trap as little air under the bladder after the cap is on. The rear master cylinder, however, needs an air pocket under the cap as it is not vented and requires the air pocket for the fluid to expand into when it is hot. This is especially important with DOT 3 or DOT 4 fluids. Double check all bolts, especially the connectors for the master and caliper, for tightness as this is a common source of air in the system. Air bubbles can get trapped in many places. Look for any loops where a bubble will hide out, particularly near the rear caliper and just before the front master cylinder where the line goes in. It's often necessary to take off the calipers to lower them and get the high spot out of the system. Let the caliper hang down with something approximating the thickness of the disk between the brake pads inserted. Turning the front wheel so the master cylinder is at an up angle helps to straighten out the loop where a bubble can hide at the top of the loop. Tapping the line with a small wrench can help to release trapped small bubbles and have them rise to the top of the system where the master cylinder is. Sometimes pressing the lever hard and releasing quickly will dislodge bubbles. With the master cylinder at an up angle try lightly pumping the lever just past the point where the bleed hole at the bottom of the reservoir is closed off (you can observe the piston passing by the hole) and release the lever quickly will bring out small bubbles near the piston assembly.
Professional tuner and 'Trials Competition' writer Jon Stoodley has very kindly stepped up to the plate to answer your questions. If you're having problems, or need some advice just send an email to Jon. If it's something that could be useful to others, we'll post it on the site.
This article originally appeared in 'Trials Competition', and was reproduced with permission.
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