1996 Tahoe Heat Problem

I recently changed the heater core in this vehicle because it was leaking I also flushed the radiator, changed the thermostat and put in new anti freeze. Now when i put the heat the air coming out is lukewarm and really doesn't get noticibly hot. I changed the thermostat twice figuring maybe the first one was defective. But no improvement. I saw 3 dampers in the HVAC sytem in the parts diagram supplied to me by GM. Is there any way to simply isolate each of these to see if this is the problem? Does anyone have an idea of what the problem may be? Thanks.
Deadhead59