brake problem

[Parrot of Doom]

I have XT brakes, and last night took the pads from the rear caliper as they were buggered (damn mud). Anyhow I didn't have any on me, so left the bike upside down for about 20 hours or so without the wheel or old pads, until I finished work and was able to fit replacements. While scrubbing the caliper clean with plain water, I pressed the lever and one of the pistons moved quite far out - I pushed it back in to fit the new pads.

There is now no braking action whatsoever, the lever does nothing. Is this air in the system, and how best to get it out - will sitting the bike upright for a few hours force the air back into the reservoir? The front still works fine.

[Gooby]

Sounds like a brake bleed is required.

[Parrot of Doom]

Same technique as a car? On a car, I normally just run a tube from the caliper nipple, into a bottle of fluid. Then keep topping the reservoir up with fresh fluid, while pumping the lever, until the old fluid has been expelled.

[mk1fan]

Do an online search for Shimano Brake Bleeding. You need a bleed kit too.

[Parrot of Doom]

You were correct - air in the caliper. Lots of it too. I bought a small couple of hundred ml bottle of oil, and a piece of tube (its about £7-8 from the bike shop). Rotated the brake lever so the reservoir was horizontal, so nothing spills out. Opened it up, put the pipe on the caliper bleed nipple, into a bag with a bit of oil in, laccy band around the top of the bag, then just squeezed the brake lever, opened the nipple, closed it, squeeze, open/close, until all the air was out, and kept topping up the reservoir.. I had about half the braking action once I'd done that, so stood the bike on its back wheel with the front wheel in the air, and left it for a couple of hours. A bit more air came out after that, and now the brake is back to normal.

I'll probably bleed it again in a week, to get rid of any more tiny bubbles. I did the front too, although there was no air in there.

Easy peasy, although it took a couple of hours - probably a few odd cavities in the caliper.