Master cylinder for sure. Many years ago I had an MGB that had the same leak. Feel the fluid between your fingers - slippery and thin? It's your master cylinder. Be careful. You will gradually lose braking pressure and eventually will need to "pump up" the brake pedal to get the brakes to bite - not a good situation to say the least. On top of this, the fluid can drip onto your pedal and your foot will slip off when trying to stop. This could be catastrophic. Replace the unit ASAP.