According to http://www.silveradosierra.com/how-to-articles/how-to-change-the-transmission-filter-in-chevy-gmc-trucks-t902.html the capacity of the transmission is 14 quarts. When dropping the pan and replacing the filter, you should drain out about 7 or 8 quarts.
For additional assistance, please find an AskPatty Certified Repair Shop at http://www.askpatty.com/search/?type=all&m=30&q=&keyword= and they can help you with your transmission.
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