77 GMC C15 Heater Blows Warm Air?

I am stumped. I can't seem to get anything more than lukewarm air out of my heater. I have flushed the radiator, put in a new 190 degree thermostat, a new heater core, new hoses to the heater, cleaned the heater housing, and made sure all the controls are working and flaps are moving. The water pump is good. The hoses get hot, and there is a new fan.