I did a bleed again last night on my XTR BL-M988 brakes and figured I'd write it up. There was a bit of finagling but it went well. This was on the bike I'd bought used at the tail end of last year and the bleed I did before was a simple one of top up oil, pump the lever and crack the screw at the bottom to expel the air I figured was in the caliper so that may be why I was having issues again. These are the steps I took and what I've done in the past on other bikes with DOT fluid.
1. Took the pads out of the caliper and pulled the front wheel off, left the caliper on the fork, cleaned up the caliper and noted the position of the pistons in the caliper
2. Pumped the pistons out about 3/4 of the way, cleaned them up, opened the bleed screw on the lever and pushed the pistons back in with a plastic shim in between the pistons
3. Attached a syringe/hose full of oil at the lever bleed port and a drain hose on the caliper bleed screw that went into a drain bottle
4. Opened the caliper bleed nipple, pushed new fluid all the way through the system until the oil draining at the bottom was clear then closed the nipple - the oil that came out was pretty dirty which may have been part of my issue
5. Pulled the shim out, pumped the lever to push the pistons out about 3/4 again, pushed a bit more oil through and watched if it was clear then pushed the pistons back in to closely match the starting position from step #1
6. Took the plunger out of the syringe on the lever and topped it up to about 2/3rds full with oil
7. Filled a 2nd syringe/hose about 1/3 full with oil, bled it and attached it to the bleed screw on the caliper with the syringe held above the caliper using a piece of tie wire
8. Opened the bleed screw a touch on the caliper and pulled a little bit of oil into the syringe while watching for any bubbles (none) to make sure no air had been introduced in attaching the bottom syringe
9. Pulled most of the fluid from the top syringe into the bottom one, watching for any bubbles (a few) and getting them into the top end of the syringe
10. Pushed fluid up from the bottom syringe to the top, watching for bubbles and seeing some come through up top and repeated this pull/push cycle 2 more times until no bubbles were seen at all
11. Closed the bleed nipple at the caliper, pulled off the syringe/hose and put the clean fluid back into the bottle
12. Removed the top syringe/hose slowly making sure to let fluid overflow the lever slightly, covered the lever bleed screw in fluid and then screwed it in slowly making sure to not let any air into the system.
I've done DOT fluid brakes this way before from when I had Avids/Hopes and used their bleed kit. I've found that the advantages are by starting with the syringe at the top you can push any crud out crud out of the system and most/all of any trapped air as well. Then by having the syringe on the bottom you can push out any air that may be trapped in the lever. The pumping action works great to get out any trapped air in the lever or caliper and any residual dirt that might be hiding in the nooks and crannies of the system. |IMHO I think a simple gravity bleed will not do this and will leave dirt behind in the system and so it's worth it taking a bit of extra time to get it right. It definitely takes a bit more than 5 minutes, but to be honest I've never seen a full bleed take 5 minutes from start to finish unless they're brand new brakes and after sorting out hose length. IMO how long takes will depend on how dirty the fluid is and of course whether there's any air in the system. I went at a pretty casual pace and it took me about 45min from putting the bike on the stand to taking it off. YMMV