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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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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
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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