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There clearly is some issue with Shimano brakes - not only from people on forums commenting but a number of bike magazines are commenting on the wandering bite point in reviews they do.
Maybe it’s similar to the problem with Avid taperbore levers on the elixirs - in a lab environment with a perfect bleed they worked ok - but in real life it was near impossible to get through to work properly for any length of time. Which is why Sram changed to the levers they use on guides / levels which have a much bigger margin for error. Other than the 1st gen ones where the pistons in the lever can swell in hot weather (also not ideal).
You dont have a diaphram issue? If shimano brakes are over filled they can push oil out the lever, potentially displacing a seal, the brake could then be bled and appear functional, but during use air would enter giving the symptoms described?
If the brake is 100% bled, then more air can be bled out after use air must be entering the system.
I had a rear SLX lever and XT caliper. It really gave me the shits as it would typically go soft after a big jump as I was about to hit a tight bend. Front was absolutely fine.
I bled the thing heaps of times and ended up swapping out the caliper ... which was the same, leading me to assume it was the lever. Sold the bike, solved the problem... for me at least.
XT M785 setup on other bike has always remained consistent. Wore out the lever pistons and replaced, wore out again. XTRs M985 have been fine, too.
I have Shimano brakes on 3 bikes - I get the WBPI on 2 of them, but only when the bike has been "stood-up" on the rear wheel for moving about. A couple of squeezes of the lever when it's down again and everything is fine. Sound familiar?
I recently sold an old pair of xt brakes. I left the hoses on the bike but removed levers and calipers. (XL frame)
To sell them I out the shorter hoses on.
Lever and caliper were cleaned and empty and pistons pushed back in.
As they were removed it was a vertical bleed.
The brakes felt amazing, better than I had ever got them on the bike with internal routed with turns etc.
If you are having a problem, try a fresh start.
In the past I have done the zip tie method on the levers over night.
I get the WBPI on 2 of them, but only when the bike has been “stood-up” on the rear wheel for moving about. A couple of squeezes of the lever when it’s down again and everything is fine. Sound familiar?
Thats air in the master cylinder, entering the system when the bike is vertical.
Thought as much - I know they're due a bleed but this isn't causing enough trouble to motivate me.
Top them up regularly (see Epicbleedsolution's "5 minute Shimano mini bleed"), and always do all bleed procedures with the free stroke screw unwound then screw it back in after you finish.
My friend who sold me my original SLXs recommended I keep them topped up for improved feel (less lever throw as the pads wear) so I've always done it with Shimanos ever since, probably 4-5 mini bleeds in the life of each set pads ...and have never experienced the fabled wandering bite point.
In the past I have done the zip tie method on the levers over night.
This works for me too I've not get some velcro cable ties for just this job. Plastic waste and what not.
It’s pretty absurd that people have to resort to regular fettling of the brakes to keep them working. I don’t think brakes like that are worth the hassle, there are other options out there.
I was a big Shimano brake fan having had a set of the original SLX which were great. As such I sold a set of formulas which had been fine for a new set of XT which had issues however much I bled them. My next bike had Guide Rs which were faultless and felt great until the Alps exposed their overheating flaws and similar wandering bite point.
Subsequently replaced with Codes which have been faultless and with bleeding edge, easy to bleed (only needed as internal routing on new frame).