Have you got the funnel thingy that screws into the resevoir on the lever? Sounds to me like there isn't enough fluid in there? Try topping up through the funnel and then pulling the lever to see if any bubbles come out. Had a similar issue with mine when I fitted them and shortened the hose - worked for me.

Yeah got the funnel and resivoir was overflowing when I put the bolt back on

Same deal with my rear brake. Got the funnel thing and all that. Mind you, they still actually work fairly well. Seem to pump up a bit in use.

Pump the lever a few times.

Does the lever feel stiffen up and stay firm? If so could just be the brake adjusting the piston position.

If the leaver pumps up hard then after some time goes soft again and requires pumping again. This usually means you have air in the system. try a re-bleed. Also check the pistons are not leaking.

Tried pumping and it never stiffened up to any satisfactory degree.

While the funnel was in place and the bleed block was in it was solid. Would this still be the case if there was air in the system?

Probably stating the 'bleedin' obvious but are the pads installed correctly in that case?

Don't use the bleed block- leave it out and use something thinner. Pump the level so the pistons poke out a bit and it should firm up.

Was also going to suggest using pads instead of a bleed block, and it's always a good idea to remove any high spots(humps in the hose)before bleeding as they can trap air.Run the hose as straight as possible with an upward slope towards the lever, and any air has nowhere to go but the M/C reservoir, and then out into the bleed cup.

I seem to be having the same problem & its doing my head in. Front is fine; always a problem with the rear.

I've taken the brake off the bike to bleed & mounted the lever on a spare set of bars in the workstand so the hose is vertical. Followed shimano's instructions to the letter (syringe to push fluid up into the funnel, then running oil from the funnel down through the caliper, knocking caliper, lever & hose with a screwdriver throughout both processes). No sign of air coming out either end & the lever feels firm (with the bleed block in place).
Refit to bike, leave overnight & the lever comes to the bars at first, but pumps up to feel firm; a clear indication that there's air in there somewhere. Bleed again (off the bike), same result; lever to the bar at first this morning, but pumps up to feel firm.

Any ideas where the air could be hiding & how to get it out? Could it be anything to do with the auto-adjustment of the pistons to compensate for pad wear? Will try using something thinner than the bleed block tonight.

I was having similar problems, again only with the back brake. I've bled and bled the brake, and i'm pretty certain I've got most (all) of the air out.

What (I think) I've found is that these are very sensitive to how they're set up. Centering the brake by pulling the lever doesn't seem to work, leaving the brake with the symptoms you're getting.

If I align it really carefully and centrally by eye, the brake feels much, much better. It's still not perfect, but loads better. I guess I've got a sticky piston (although I'll be damned if I can tell which.)

Worth a try?

(I love these brakes when they're working, but the rear was doing my head in to the point where I was considering getting a set of Avids (of all things) to replace them. Going to see how they go with centering them this way.)

Will try using something thinner than the bleed block tonight.

Careful with this one. I tried this using an 8mm allen key. It moved, I didn't notice. I pulled the lever fairly hard, the piston came out wonkily and the seal blew. That's the second piston seal that's gone for me on these brakes in 3 months (the first went while riding) - I think they might be quite fragile...

If I align it really carefully and centrally by eye, the brake feels much, much better. It's still not perfect, but loads better.

I've centralised the caliper as per the video by the Hope guys to ensure the pistons are working an equal amount. Initially it feels perfect, but when left for a while the lever comes to the bar & pumps up to feel firm. This isn't set-up; there's air in there somewhere.

And I've popped a piston out in another set of brakes before; not a mistake I'll make again!

Fair enough. Personally, I'm convinced it is a set up issue, as mine have been bled and bled and bled with no real change, whereas paying really careful attention to set up seems to have sorted it. Maybe give it another check?

Just bled with the pads/spacer instead of the bleed block.

Feels firmer than before, but still noticeably softer than the front.

Will find out tomorrow if there's been an improvement as didn't manage a test ride.

I had a similar problem, assuming you are bleeding from the caliper up to the lever with a syringe....bleed as you did, remove funnel etc....refit screw. Then push a small amount of fluid into the caliper then seal the bleed nipple. The extra fluid seems to sort it.

I had this exact issue. To sort it is simple - drop your rear wheel out of the frame and gentle squeeze the rear brake lever two times - remember, gently and not all the way back to the bar. Then refit wheel and all should now be ok. Its to do with pad wear and the system not being able to automatically account for the resultent increase in pad movement required to make the pads hit the rotor.

I'm having the same problems, ok with the yellow block in but crap when the pads are in. Even tried pumping the brakes with the wheel out.
Front is spot on.
Very annoying!
Anyone else sort the problem out yet?

Same issue here. Front is spot on, back is spongy. I've trimmed the hose and bled twice to be safe and there's no air coming through. With the block the rear is solid but put the pads in and it's cr@p.

Has anyone tried alternative pads (specifically the Superstar ones)? I managed get some oil on mine (I think) and I wouldn't mind trying an alternative.

I don't have M785 brakes but I think that the levers have a bite-point adjuster. It there any chance that these are set differently for the front and rear. I'm not sure how they're meant to be set for a bleed but presumably they slightly alter the volume of the resevoir.

Failing that a bottom up, syringe bleed sorted the issues I was having bleeding my rear M595 caliper.

I've bled my newish XTRs 3 times now and they're still pumping up, front and rear. Worked great for a while, then started pumping up and I can't sort them despite all the right tools, top / bottom bleeds, even reading the instructions... Gave up and fitted my old XTs )

Bite point adjuster only looks like it affects the lever position, not the piston position. I backed off all my adjusters last bleed, no difference. Puzzled.

but when left for a while the lever comes to the bar & pumps up to feel firm. This isn't set-up; there's air in there somewhere.

Interested in this myself.

Get a generic bleed kit that had a hose a little thinner than the Shimano hose and has to be forced onto the bleed nipple on the caliper. 1/4 or so fill the syringe and ensure that fluid goes right to the tip of the hose.

Screw on the yellow funnel (or another syringe from the generic bleed kit).

Now pull fluid carefully through from the syringe mounted on the caliper. This should get rid of any air near the caliper. Once the air is gone force fluid back into the caliper to the point where the Yellow funnel or Syringe on the lever starts to fill.

Close the bleed nipple on the caliper and remove the syringe.

I used the threaded syringe at the caliper and gently pushed some fluid into the lever too until it started to seep past the seals. Remove funnel or syringe drop a little mineral oil into the hole and refit the bleed screw.

It was something along these lines that sorted it for me on my ones. It's kind of a mix between how Avid suggest bleeding and how Shimano suggest bleeding.