If you haven't got compressed air to use you can do the rope trick. Bring the piston up to TDC and then push some cotton clothesline into the cylinder until you can't get any more in. Then go ahead and change the seals. Pull the rope back out and do the same to the next cylinder. Be sure to have a long enough rope so you have a long tail hanging out of the cylinder when it's full. You don't want to have to pull the head to get your rope back.