I was riding in the rain today, and I noticed a little hole on the side of the cover for the brake fluid reservoir. Right on the seam where the lid/cover for the reservoir bolts on. It was a little hole that looks rectangular, and when I pulled the brakes, a little bubble would appear there.

Any ideas what this is?

Thanks,
lolumad

 

This little cutout is called a vent trail. It allows the hydraulic system to work properly by venting excess pressure build up in the master cylinder. Normal for it to bubble now and then. Just keep the vent clean and open. OPT

 

Thanks, glad I don't have to worry about that!