I wave these brakes on a trek 3900 I Rebuild from leftover parts from my other bikes+ some new ones, So this is a bike I don’t ride often, But 2 years ago I rode these the bike with these brakes the whole winter, in various temperatures down to -11 Celsius ,And of course I used the brakes very often, but I noticed oil leaking trough the master cylinder on the left/rear brake, So I checked it it was the Torx screws that were the issue, but that was not the case, the threads I in the master cylinder were destroyed, they were polished to powder, so the screws did not hold the seal and the lid in place properly, so I ordered a new complete lever, but after only 2 trips about 10 Km each trip, the part which pushes the got scratched,and this was not caused by a crash, these brakes are just too weak.
The lever body is flexing when you press the lever blade all the way, Which I probably why the part that pushes the piston in gets scratched, and why the threads fail, and why the brake leak.
But there is significantly less flex in the 396 than m615, the m615 flex about halfway,and the m396 flex at near to all the way.
I had not had any issues with the seal in the calliper, or the calliper, But I have not ridden the bike with these brakes as often so it’s most likely why.
This is a way too flexible & weak brake, I strongly advise against buying these.