I have fished broken keys out of locks with a pen vise. The cylinder has to be in position for the key to come out if it were intact. Screw the bit into the key as far as you think it will go without exiting the far end. As you do this, periodically try to pull pen vise out to see if remainder of key will come out. I've usually needed 3/16 inch of the bit in the key before it obliges. Vacuum cylinder afterwards to try to remove any brass shards that may have fallen into the cylinder.