overheating....but no heat?

[als70chevy4x4]

My 3/4 Ton has a problem i havent seen on here yet.When I bought it the heater core leaked and didnt throw barely any heat when fully warmed up.I changed it out and still no heat.Then thinking the rad wasnt up to snuff (green color around the sides) grabbed a rebuilt one out of my parts truck(2 core instead of 3)same problem.When I've got my camper on or a load of wood climbing hills I can watch the temp rise steadily.Talking with some different mechanics they have mentioned air lock???I've flushed the system twice and still no luck.Anyone out there had this problem and corrected it?Help!