Dont worry about it. There is some air in the joint and when you push the caps on the air pushes some grease out of the seals. It will loose some grease but not that much. The center of the joint has grease packed in it and will move towards the caps when it spins
Were the cars in you sig really your Grandpa's and great Grandpa's. Post some pics of cars if you get a chance. Thats cool