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I have a bike with Avid hydraulic brakes. The levers feel loose and at the end of pushing it, there is a "soft" feeling (like when V-brakes pads aren't adjusted properly).
As I understand, it's air inside the pipes. Is there something I can try to do to fix it without opening it / refilling liquid?

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If there is air in the hydraulic lines they need to be bled. Sometimes you can trick the system into bleeding the trapped air without doing a full out bleed procedure, but the only sure fix is to do it the right way. – Daniel R Hicks Jun 6 at 15:28

Hydraulic brakes develop air in the lines as they're used ( heat / chemistry are to blame ). The only way to remove that air from the system is to properly bleed the brakes. Trying to find a shortcut around this important procedure probably won't help you any, and worse case scenario you could kill yourself or someone else when your brakes fail.

With this in mind, you should probably pay a professional to bleed them properly if you don't know what you're doing. Also, the Avids that use DOT fluid have a habit of degraded performance over time, regardless of there being air in the fluid. If yours feel soft even after a professional bleed, I would recommend upgrading to a higher quality hydraulic brake ( one that uses mineral oil ).

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Does it mean, that even fully replace the fluid will not help with degraded DOT? – Alexander Jul 2 '14 at 0:25
1  
If you fully replace the fluid, the fluid is essentially new and the performance degradation goes away. It seems like a lot of the Avid's are spec'd with DOT 4 or DOT 5.1 - DOT 5.1 is backwards compatible with DOT 4 (at least for cars) [ but DOT 5 is something totally different ] and might hold up better. But you do have to re-bleed the system. I doubt the fault is due to using DOT, though, and more likely a flaw in how Avid designed their brakes. – Batman Jul 2 '14 at 2:54

You need a kit to bleed that brake. Disc Brake Bleed Kit
It is one of those tools/kits that as an individual it is not worth buying. Even on Amazon the kit it $40.

A (quality) bleed replaces the fluid.

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In this situation a full brake flush & bleed is always best, but there is something you can try that may get you out of a bind. Orient the bike so that the reservoir is the highest and caliper the lowest, and such that the brake hose attachment on the caliper faces upward. The cable should also be inclined all the way with no dips along the way. Next, tap the caliper and brake hose moving up towards the brake master cylinder using a small wrench or the handle of a screwdriver, etc. This can sometimes entice air bubbles to move up to the master cylinder. After repeating this a couple of times, squeeze the lever multiple times. You may have to reorient the bike or move the brake on the handlebars such that the air bubbles will move from the hose into the cylinder and up the reservoir. Rinse and repeat.

Important note : If you have had air in the system in warm/humid weather, you may have gotten water into the system as well, water that is dissolved in the brake fluid and does not come out with the tapping procedure. Lever may feel fine in the garage, but water boils when heated up and may result in brake fade when braking down a long hill etc. Always do a brake flush & bleed when possible.

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