Don't automatically assume the master cylinder is leaking just because you have fluid in the cockpit. It's the most likely path, true, but I had a situation on my 9A where I had an air bubble in the line and had the reservoir completely full - and every time I climbed up high (mid to high teens) it would burp out a few drops. The fluid will come down the line from the reservoir all the way to the connection at the master cylinder and drip on the floor - looking for all the world like a cylinder leak. Took me quite a while to figure it out and purge all the bubbles out, and started keeping the reservoir just 3/4 full, then just like magic no more leaks...
I didn't identify the real culprit until trying to clean up fluid one day with some ultra-cheap paper towels that were thin enough to read through - I ran one down the line from the reservoir and it came off visibly wet - but you couldn't see it on the line in low light and a good paper towel might not have showed up. Try using a coffee filter or plain white paper on that upper line to see if it's wet.