A high volumn low pressure pump can be used to blow the ballast. I use a nicad battery model.
Insert into the vent port in the anchor locker. It's helpful during the process to hold down a slight bit of pressure on the valve shaft to seal the washer and keep air from escaping. Use of the powered pump of course makes this easy to do... start the pump, then go to the valve... when the battery is wore down after about eight minutes the tank should be empty.
Hint: Weight the boat to starboard slightly for this operation and then the bubbles will migrate to port and collect in the center board trunk and can be heard bubbling in the c/b cable tube.