I had this same problem on my '51 after I took it in for service one time. There was something screwed up about the master cylinder when I got it back. Drove me crazy for a few months with the same symptoms you mention. It had already been rebuilt numerous times, so I ended up buying a new one, and all was good after that.
I know you have a NOS one, but maybe it would be worth taking it apart and checking it out.
Kurt
