I have a problem bleeding Shimano XT brakes: The oil can come down if I open the system at the bottom, and the oil drips down. But once I connect the syringe, put the little plastic cup screws on the top, then I cannot pump the oil toward the top. I have done several tests, tried to undo the bleeding screw at the bottom on the caliper of increasing turns (1/4, 1/2, 3/4...until the oil comes out from the screw sides (too unscrewed). I have as well tried using a différend bleeding screw, and the oil pass through nicely. I have opened and cleaned the master cylinder at the lever, so I know all is good. All I can think is it is a hose and oil/pin problem. What do you think? Ever seen anything like this? I should mention that I have worked as a bike mechanic for the past few years... Thanks Paolo
The lever reservoir has a tiny hole to let fluid and air pass in/out of the system. This hole gets closed off by the master piston when the lever is pulled.
I've seen similar behaviour to yours once before when the brake had been reassembled (reach was too tight or too loose I forget), such that the master piston was out of place and the breather hole was closed off, preventing me from pushing fluid up from the caliper end. Fluid seemed to flow down ok but that was probably merely residual in the hose. I experienced the same problem as you describe and tried the same steps focusing on the caliper end first before realising what had happened. Bike owner confirmed messing with the lever before having initial problem.
Caveat: this was on a cheaper/simpler brake, BL-M315 and the reach adjustment on yours might not have the same effect, but I would try resetting the reach adjust to see if it has an effect.