Ok, sorry for the long delay in getting back to all with the resolution of this problem. The short answer is that it was in fact my master cylinder. It has been replaced and all is great now. I'm all set for a summer of top-down enjoyment!
Thanks, M60A3Driver for the well wishes! I had the other knee replaced in September and all is good with me too.
I'm glad to hear that your surgery went well and that you found the issue with your brakes!