I've had the same problem. I first tried to clear my form data by clearing the model-object upon closing my modal window, just like you, but that only worked for when I submitted the form, it seems. When cancelling, it doesn't work! (Even if you explicitly clear the object before hiding the popup, it will not work)
I eventually fixed it by doing this:
$scope.newTask = function() {
$scope.task = {};
$scope.taskModal.show();
};
This way, every time the window is loaded, you clear the model. So the trick is not to do it when submitting data, but when opening the modal window. That did it for me at least.
Btw, I also needed an edit function for this same modal window, so I do this:
$scope.editTask = function(task) {
$scope.task = task;
$scope.taskModal.show();
};