Brake behaviour weirdness help

If brake is all built then tie brake lever to bar and elevate the lever so it is very high (so hose is as vertical as possible.

Leave lever tied to bar for 24 hours. It should help move any trapped air to the lever and you could then remove that.

Might not help but it sounds like there is still air in the system.

Lever applies lots of brake pressure with tiny movement, great!  But with pads and disc back in, lever goes to bars, pistons hardly move.

How are you measuring this considering you have no disc and pads in the caliper?  Finger pressure against the pistons or have you put a pad spreader or similar between the pistons to check the brake operation?

If the pistons are sticking, they won't pump out to meet the disc after being pushed back, so you'll get a floppy lever, but they will clamp something wider such as a spanner. You can help them out by holding one piston in whilst pumping the lever but they'll need cleaning up. 

The initial problem of the lever not wanting to return does make me suspect that the lever/master cylinder is knackered though.

Does sound like a lever problem. I've known seals swell, springs fail and piston bored corrode.

From the small press big pressure to large press back to the bars it could be an air in the system job.

But combined with the non returning lever it sounds more like a lever problem. The remedy is to buy a new lever (all shimano are compatible with all shimano calipers, I believe). They come pre-filled. If you're very careful you can install then just top bleed a few bubbles out.

The final recommended step is to take the old lever and hit it repeatedly with a lump hammer (eye protection recommended) whilst muttering the vilest language you can muster.

@spooky Yes - using a yellow block spacer for bleed fill.
The lever does return after going to the bars having done the bleed.
I’ll try the zip tie and vertical thing today while I look for a new lever.
This may or may not result in me looking for a new bike with full justification

Before you buy new bits or try bleeding again etc. have you tried adjusting the lever to the full out (furthest reach) position so it can move more brake fluid per pull? This can help overcome sticky pistons to move them out to make contact with the disc. Then you can adjust reach to something more comfortable.

Would a Zee lever be compatible with the M6120 caliper?
I know XT/Deore all mix and match, but not sure on Zee and i can get a Zee.

Isn’t Zee just a Deore lever on a Saint caliper?  

Grit embedded in the lever piston with Shimano brakes. I'm never sure if it comes in straight from the outside (from hosing down or sticking the bike on a car bike rack) or from the fluid line (the bevelled edge of the piston is a perfect trap for any crap but I suppose it makes factory assembly easier).

Swollen  seal in the lever - when bleeding lever pulls to the bars then it's solid immediately after