Brake bleeding seems to be a popular topic here and I’m a bit confused about the best way to bleed my Shimano LXs. These are my first hydraulics (BB7s until now) and I am a bit of a numpty, so I’d like to get my head around them.
It seems to come down to the use of the bleed nipple. The instructions that came with the brakes concern brakes with no fluid in (as it assumes that the hoses, levers etc. have to be connected). It talks about opening the bleed nipple, filling the reservoir and cycling the lever, whilst refilling the reservoir, until fluid comes out of the bled nipple (presumably this primes the system with fluid). It then goes on to talk about closing the bleed nipple and cycling the lever whilst watching for bubbles appearing in the reservoir (I assume that this is bleeding). Then you’re supposed to apply the brake and then rapidly open and close the bleed nipple 4 or 5 times to release any bubbles trapped in the caliper. Now this makes sense to me as air is less dense than oil and so is likely to clear the system by going up. However, my Zinn and the art of mountain bike maintenance book and DVD give a different method, as does this month’s WMB magazine.
With this method you pop the tube/bag on the bleed nipple, open the nipple and then apply the brake pushing fluid out of the nipple. You then close the nipple (with the lever applied to avoid pulling air in through the nipple), release the lever, top up the reservoir and repeat (open the nipple, push fluid out etc.) the job is supposedly done when the fluid in the tube has no bubbles in it according to WMB, whilst the Zinn DVD simply repeats the process twice. Now as I understand things you can’t just push air out of the nipple, this approach just seems to purge the fluid and air from the system whilst pushing new fluid in (does the Zinn DVD only cycle twice as two cycles will clear the system whilst filling it with new?).
So, when the day comes to bleed my brakes (hopefully not for a while) which approach should I use? The Shimano approach seems to suggest that you can bleed air from the system without replacing all the fluid and I’ve heard that tying the lever the the grip overnight can bleed the system, or should I be using the Zinn/WMB approach and completely replace the fluid (if I’m understanding the technique right)?
