Heater doesn't get very hot

[Skunksmash]

Another heater thread I guess. My 87 heater seems to have lost a lot of its functionality. The truck has to have been warmed up all the way for at least 15 minutes before the heater will barely start to work. It always had a good heater up until this winter.

I made sure its full of fluid, and it seems to be. I did just replace the radiator, but I drove it several times and filled it up each time it sucked more fluid down. So there should be plenty of fluid. I think there can be air in there, but I'm not real sure how to tell if that's my problem.

Once the truck has been running for about 20 minutes, the heater will finally start to blow somewhat warm, but it never really blows hot like you'd want it to. There is no leaking going on in the cab floor. If it is the heater core somehow, are they difficult to replace in these trucks?