Hello, check if you put the beveled side on backwards. It's the bevel that allows for the piston to return after the pressure is removed. The seal deforms when the piston pushes out. The rubber then wants to return to normal. If the piston is pushing with the direction of the bevel then the piston is sliding along and dosen't deform the seal. And it will leak. If there's is no return motion that's a very small movement of the caliper when the pressure is released. No return motion is a sure indication that the seal are on backwards. You have to get a magnifying glass to examine the seal, to see the bevel. The outside edge of the seal should be square cut, that goes into the caliper groove. The inside of the seal one edge is slightly longer than the other, this gives it the bevel, which you can feel the peak edge with a finger. Now if the piston slides with the bevel, that is to say pushing the pointy end out, the seal won't deform. Put the raised, pointy, end so the piston pushes on that side first then the seal will deform over itself. Then when you let off the brake pedal the pressure is released and the seal, being made of rubber wants to go to its normal and pushes the piston back, and with no leaks. If the caliper is letting the fluid out over night, maybe slightly, brake fluid evaporates, put a paper towel there if it's wet or has stains after sitting over night, that's the kind of leak from seal on backwards. Don't forget to clean the rotors with brake cleaner. If not riding for some time then apply the brakes every so often to keep the seal lubricated and supple, preventing that kind of leak.
