I've experienced that on the rear brake of my '15 SGS. I'm pretty sure it happened because I put a little too much fluid in the reservoir after bleeding down the brake and when I inserted the diaphragm, that made the fluid rise even higher. BTW, the cover screws need to be torqued to 13 in-lbs or the equivalent thereof. Once that's done, spray some brake and parts cleaner on a rag and wipe the perimeter of the reservoir down. That took care of my problem and it's been dry ever since.
