An ancient denizen of the 'net, T.Alan, coined the phrase, "It's Impossible To Balk To Second Base." Unfortunately, some umpires (and coaches) believe it is impossible to balk while in the wind-up. I have been hammered with, "He was in the wind-up, ump, he can't balk!!" by allegedly intelligent coaches.
As pointed out already, it is a balk to simulate the pitching motion while not in contact with the rubber. It is also a balk to begin a motion that commits the pitcher to pitch (or throw to second) and then stop.
Partially in defense of the umpire, what you described would have to be seen to call correctly. After all, we all know there are at least three sides to each story.
That said, if your description is totally unbiased and 100% accurate, we have a balk.