Shimano Brake Bleed, lever won’t pump fluid from Res

I’m trying to bleed my XTR M9020 brakes, front went well but both brakes are needing a lot of pressure on the syringe, pretty sure this is just because the syringes are from a DOT kit and the rubber plunger is sticky, but they are doing the job.

However, if during bleeding I momentarily crack the nipple on the caliper as some guides suggest before finishing off the lever end, on subsequent pumps the lever is not refilling the system despite the funnel being attached with spare fluid.

And again, if I finish up and put the pads in, the lever is not pumping the pistons out enough to engage the brake despite having brand new, full thickness pads. The lever goes to the bars after a few pulls and only pops back out when more fluid is introduced from the caliper.

The only other factor with the lever is the master piston is a little lazy to return, tried to find a guide but the two links I found were broken as they are a couple of years old. Apparently there is a spring buried in there? Had a gritty weekend away last week and as the pads wore, the lever went to the bars.

Help! Thanks

Sounds like the master cylinder piston is stuck/not returning, and masking off the port into the reservoir.

coatesy’s theory sounds plausible to me.

It sounds to me like the system is underfilled. It’s often hard to get all the air out of the reservoir. This will cause problems when the pads wear or if you lay the bike on its side.

Yep the immediate suspicion is lack of fluid…but there was a half full bleed cup on the lever :)

Found a guide, sounded fiddly but not as bad as it sounds https://bicycles.stackexchange.com/questions/81986/how-do-i-rebuild-shimano-xtr-brake-levers

Master cylinder needed fluid adding at caliper to remove, the big spring behind it wouldn’t push it out. It had some gunk on it so hopefully with a clean that will fix it.

Not how I was expecting to spend today but at least it coincided with crap weather!

Mmmmm. Spring and piston back in, seems like the piston has gone all the way in and the spring is not pushing it back. System has no fluid in it but expected it to move bank and forth?

Left the brake hanging whilst debating next steps, the piston pushed out again, I cycled it a few times (sticky) and now its stuck in again. Perhaps I’ve pushed it beyond its normal operating depth…

Confused. Fixed the lazy piston and the lever now returns nicely, (smear of grease on the piston did it) re-bled, lever nice and firm. Remove bleed block and it just won’t pump the pistons out…they move, marginally. Grrrrrr :(

Gave up and bike the bike back together for a professional to look at.

Back wheel in, lever still coming to bars. Front wheel in, moved the bike around and realised rear brake now working fine! But take wheel out and it refuses to pump the pistons out (I’d already cleaned and lubed them before pushing back)

Going to ride it, hoping I can forget about it but suspect as the pads wear the lever will move in…