Having problems here and could use advice from experts. Had to replace the heater core in my 99 Disco. Have the new heater core in but can't get the o rings to seat properly. Tried to put the o ring in the heater core first, and then the pipes won't push all the way in. Then tried to put the o rings around the pipes, but then they don't seat properly in the heater core. Does any one have any advice? Have tried lubing them with soap, then tried a small amount of vaseline, but nothing is working. Any other ideas?