For the last two months and less than 600 km of riding I have been using a pair of new Shimano SLX BR-M7120 "high power" 4-piston hydraulic disc brakes. Yesterday I have noticed that the bite point of the rear brake started to wander, i.e. the lever travel required for brake engagement changes.

Specifically, when pulling the lever a few times in quick succession, i.e. when pumping the lever, the travel shortens considerably (up to 1.5 cm at the finger position, depending on how fast you pump), so that the brake bites sooner, with the lever further from the handlebar than normally. The lever never sinks closer to the handlebar than normally, i.e. the bite point always wanders to the outside.

The brakes have been bled very recently and the lever does not feel soft nor is the braking weak. Combined with the fact that the lever never sinks, I am not sure this is due to air in the system.

This is quite a frustrating and unsettling issue in general and especially when hurtling down very steep and rough terrain (which these brakes are meant for) where it is dangerous both to go too fast or too slow, one of which you are likely to do if you cannot tell when your brakes will engage. So as the question says, does anyone know why this happens and how to fix it?

Some additional info:

  • front brake is also affected by this issue but to a far lesser extent; the lever has to be pumped faster than it would be during any realistic trail riding situation
  • problem first occurred in wet and cold weather (just above freezing), however I've ridden in the same conditions a few times before without noticing it
  • the brake is not damaged and, apart from this issue, works great
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    This was also a common issue with previous generations (2-3 years ago) of XT brakes. You might be able to gain some insight googling for those. I'm afraid I don't recall reading an actual solution to the problem though – Andy P Dec 2 '19 at 13:23
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    A few questions to rule out common causes of similar issues: (a) are you turning your bike upside down or on the side for repairs or transport? (b) do you experience a lot of break pad wear? (c) do you notice frequent brake rub, is your brake rotor bent or wobbly? – gschenk Dec 2 '19 at 16:00
  • Did the brakes do this before bleeding. Have you tried bleeding them again? (You can probably guess I am thinking its a bleed problem) – mattnz Dec 2 '19 at 20:04
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    So the answers to your questions are as follows.I am not turning my bike upside down or on the side (except when I crash that is, when I do both :D ) – Mick Dec 3 '19 at 9:09
  • The brakes and pads are almost new, there is no excessive pad wear. The brakes do not rub. The brakes did not do this neither before nor until several hundred km after bleeding. I bled the front one once on the lever only, and the rear one once as a full bleed and twice on the lever to get a firmer feel. So they were bled just fine, no issues whatsoever. I will try bleeding again, but I somehow think that's not it. – Mick Dec 3 '19 at 9:19

I will give a tentative answer and expand on it as I use the brakes for a longer period.

Firstly, this is not a bleeding problem. I verified this by thoroughly bleeding the brake yesterday using fresh original Shimano mineral oil, after first draining the system to make sure I really do use the proper fluid in good condition. I also took the opportunity to clean the pistons and "lube" them a little with the braking fluid, just to eliminate dirty/sticky pistons as a possible cause of the problem. The bleeding was done at room temperature and the results were excellent; great lever feel, sharp engagement, consistent bite point position. However, after I took the bike for a test ride through the neighbourhood, with the outside temperature just around freezing, as the system cooled to the outside temperature the wandering bite point reemerged, i.e. the lever would "pump up" (engagement would begin earlier). The corresponding effect, to a smaller extent, emerged at the front brake as well.

Given this test and the information I have gathered at various places on the web, I conclude that the cause of the problem is the high Shimano mineral oil viscosity at low temperatures (viscosity being, in layman's terms, the resistance of fluids to flow freely; water has very low viscosity and flows easily, while e.g. honey has high viscosity and oozes slowly), combined with the small orrifices of the Shimano levers' inlet and compenstaing ports through which oil flows between the master cylinder and the brake fluid reservoir. Namely, instead of the oil flowing to the reservoir through the compensating port as the pistons are retracted after braking, the oil stays in the main line, so as the lever returns the pistons do not yet retract fully, meaning that on the next braking they are extended closer to the disc and therefore bite sooner, with lever further from the handlebar. If given more time, the oil slowly flows back in the reservoir, the pistons retract fully, and the bite point returns to its usual position. Hence, the bite point wanders to the outside by amount dependant on how often you engage the brake; at low temperatures, you can pump up the lever so that there is almost no free stroke before engaging.

The solution I have read about and intend to apply is to swap the original Shimano mineral oil with a low viscosity oil that has small viscosity variation with temperature (i.e. a high viscosity index or VI; Shimano's VI is pretty low, apparently, meaning it gets "sticky" at low temperatures). I believe this nullifies the warranty, but I have found there are many people, especially in Germany, who have been doing this for years using one particular motorcycle fork oil. In fact, this is so widespread that this oil, Putoline HPX R 2.5, is considered a benchmark on their forums, and if you try to buy it on German Amazon, 7 of the total 8 reviews rave about its use in Shimano brakes, while the eight also lists this as one of the uses it is good for, while Amazon auto-suggests Shimano bleeding funnel as an additonal item to buy with this moto fork oil (!).

After I actually try this, I will update this answer with a report on the results.

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