It could be the grommet that the reservoir mates to the master cylinder has gotten old and needs replacing, but it's also possible that the cylinder itself has finally gone bad. I had to help a buddy replace one this year, they are good parts and don't fail often, but they still do wear out over time. Luckily they aren't too expensive and are easy to change. It may be a good idea to just replace the master cylinder and then bleed the brakes, last thing you want going out on a hill is the brakes.