Yep, I moved all my vacation to this past January, PD'd my only awarded trip and then took out 20 hours from my bank (I had zero time in it so I went negative, too) to get back to a descent credit for the month. Did this to get the whole month off for the birth my kid.

I think the limit is the lesser of 82 hours or ALV + 5 to take a withdrawal for pay purposes, even if it takes you negative.