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