^ I have found that if I "over fill the system" by using the Shimano funnel to add fluid to the levers with the wheels installed on the bike in order to do the popular "5 min Shimano lever bleed" trick, then the variable dead band change is at its worst.
So now I only add fluid and/or "flick the lever to bleed it" after the pistons have been pushed back into their bores and then the yellow bleed block installed.
I believe this insures the correct fluid level, and what Shimano does during manufacturing.
While that hasn't completely fixed the issue for me, it does reduce it by a noticeable amount.
I think you are on to something here. I read in another thread someone had the same experience and observed that with the topped off reservoir, the pistons have a more difficult time retracting. It kindof makes sense, I suppose, the fluid seems to thicken when it's cold and I could see a situation where it can't retract back into the reservoir fast enough when there is excessive oil and not enough space.
I have two other friends that use the same brakes and haven't had this issue, one said it happened a few times when she first started using them, then stopped. Another said it was never an issue.
I experimented with the process that the Syndicate mechanics used and published on YouTube a few months back, which is basically a gravity bleed with the bleed nipple removed entirely, followed by what amounts to a bleed using new pads or the bleed block (they used new pads but I think the difference is negligible). They also bleed with the freestroke fully out, then close it while doing the lever portion of the bleed. I tried this and, so far, there has been no issue and the amount of stroke change I get is negligible even in colder temperatures.
The worst time I noticed this was when I did a full Shimano bleed, using their instructions, followed by the reservoir top off technique with the pads in. I think people use this to get a tighter lever feel, but it seems odd to me that it would be necessary to do as the pads wear, that'd be a pretty big design deficiency IMO. Later, I went to bleed them and removed the bolt on the lever and the system was pressurized. The extent to which I had lever issues was the highest at this point.
I have a spare M8020 rear that I will experiment with this on once it gets cold enough again.
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