I have a Shimano XT BR8100 system, all was good until I did some maintenance. Probably I did something wrong, trying to understand what. Did a normal procedure to replace oil and pump out air.
Visible problem is that they squeak at very high tone when I put medium to high push on a lever. And I see significantly reduced braking effect. In practice, I can't lock the front wheel with that.
I've tried to exchange front and back caliper, and the problem also swapped. This somehow shows the problem is in caliper/pads.
Purchased new pads just in case - does not help.
No visible oil leaks (or I don't know how to search for it).
Another funny evidence: As soon as you put some water on a braking disk and on a caliper - the issue gets fixed temporarily! Full power to lockout the wheel is restored, I can again easily lock the front wheel on any speed. And the screaming sound is also getting fixed (until the water dries out).