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.
I have used these brakes on 3 bikes, some more than others, the one That the threads for the Torx screws failed, was the one I used on my main bike.
The brake lever body/Master cylinder flex when braking harder than 50%,which is why the plastic lid & the separator & the Master piston unit to bend or get damaged, and cause leaks, so the lever will be soaked in oil, and oil will spit out through the middle where the plastic lid is.I compared the flexing in the levers on m396&m615 and there is significantly more flex in the m615, if you press the lever blade half way you wont see flex on the m396,but on the m615 you will.
Leaking caused by plastic cover bending upwards, and front and the back is also being pushed upwards but sideways too, causing a leak.
temperature changes, will cause this, a minor hit can cause this. the design is clearly flawed.
I had to replace one master cylinder due to failed threads, and I had the leaking issues with the new one too,but with a set of pliers it was fixed, by pressing the cover into the master cylinder until it was straight/flat against the master cylinder,but this is pointless to do, you are better off buying a mechanical brake or a proper hydraulic brake.
Impossible so far to get placement seals/grommets/diaphragms, or just the master cylinder separately.
I don’t know what caused the threads to fail, it was certainly not over torquing the screws,I don’t know if its caused by fluid pushing the parts and girding the threads apart until they are so polished that they cant hold the cover, maybe I don’t know, I certainly was not a crash that caused it,because I never crashed with this particular brake.
Hinged clamp,no need to remove grips & shifter.
the levers are easy to bend back if you bent them,which I have done on 2 bikes so far.
good braking power,at least if you use Metal pads, ( if its wet or ride a lot)
it’s not too expensive
it’s not too heavy
no need to take off grips to take off the brake, because of the swing arm clamping mechanism.
The Brake Lever will bend if you crash into a tree(before I upgraded the handle bars i was used to ride on the fuel ex 7 with 720 bars),at low-speed,don’t just let the bike fall on the ground,it can bend the brake lever.
You can of course bend it back with proper tools and technique.
M615 Piston seals:
IUY 21.1 25 3.19 78 NOK
The Both brakes are too fragile.
Because the brake lever body/master cylinder is way too flexible and that there is a palstic lid,instead of metal lid,which causes the brake to leak.
I have seen a lot of reviews claiming these brakes are a good value at a low price point.
I agree with the braking power for the money is there, but not the reliability, these brakes are too fragile.
same goes for the other brakes with the similar design, for example, M396, M395,but at least it’s possible to get seals for M396, M395.