Why can't I bleed my Shimano brakes?

My guess is that there was air in the reservoir and when you removed the bleed block and refitted the pads, there wasn't enough fluid in the system to pump the pistons out. Shimano brakes can be fiddly to bleed, it's really easy to end up with bubbles of air in the reservoir which then causes wandering bite point.

It's also not impossible that the master cylinder seals are shot, but air in the reservoir seems most likely to me.

Give up. I can bleed any brake and get a rock hard lever feel. My XT 8120 rear is straight from the devil. I thought that the wandering bite point was just something that happened to other people, I’ve invested too much time in bleeding the bastard in a million different directions, angles. I’m scared to pop a wheelie anymore. I know it’s an air bubble in the caliper but quite frankly, it’s defeated me.

The Guide RSCs on my 2015 Capra have been flawless, they just work with loads of power.

The Code RSCs in my lad’s 2018 Capra are insanely good.

The Guide RSCs in my 2017 Tues have been flawless though years of punishment at Leogang.

Try rotating the bike in the stand to get more vertical in the hose, particularly for rear, that might get the air out of any nooks

Detach calipers from mounts and tap and move them around to see if you can persuade the air to move to the hose (less wiggle if internal routing)

Try the epic bleed kit w 2 syringes and create vacuum at the lever to persuade the air leverwards (a la SRAM bleed)

@thols2

It’s also not impossible that the master cylinder seals are shot, but air in the reservoir seems most likely to me.

They're brand new brakes so hopefully not the seals.

The 2 syringes was going to be my next attempt.

I had something similar with my deore rear. Solid with the bleed block in, once 50% worn pads went back in it had to be 'pumped' up to work whilst on the trail.

I tried the Jason Marsh bleed method and it's worked a treat. Back to solid lever.

The only bleed issue I have had was fitting my 4 pot slx caliper. Nearly gave up on it as it was so spongy like you say. I eventually got it OK by not using a bleed block to allow more fluid in and having pistons close together at the start, using Marshys method posted above. my only guess was there was air in it somewhere/somehow.

Thanks all.

Will try a narrower bleed block and see if that makes a difference, and order the epic bleed syringe.

Just to clarify these are new brakes out of the box with new pads.

I wouldn't order anything before trying the Marsh method. May well solve your problem for free!

If the Marshy method doesn't work nowt will, follow it to the letter, don't bother with the bleed block, Marshy doesn't.

I've had some success clamping the lever on and leaving the bike vertical over night with the cup still fitted. The idea is that the micro bubbles find their way out. 

But I'm still struggling with a caliper I replaced the seals on.  

i never use a bleed block

just push oil up from the caliper into the funnel up top

granted when i change pads i usually have to let some oil out again, but that's just another bleed effectively

Will give Marshy's method a go 👍

Just an update on this - turns out it was sticky pistons!

Gave them a lube and all sorted.

Don't know why I didn't check them earlier......

I used to think wandering bit point was just something other people suffered from. I have had pretty much every iteration of Shimano brakes and never experienced it.

But on the latest M8100 series had it on the rear and to a lesser degree the front. worse the colder it is outside.

Tried all the various bleed methods (marshy etc) even had the calipers apart etc, check for sticky pistons etc, too the brakes off the bike to allow more manipulation of angles etc. even degassed the shimano fluid.

In the end just put this in and issue is resolved.

Not sure if it works as it is clearly less viscous, or as it less viscous it is easier to get a proper bleed, or a bit of both.