1994 Chevy Caprice not enough heat

Hello my problem/question is after the car is at operating temp. I turn on the blower and it blows normaly(hot) for about 30 seconds then cools down some. Ive flushed the whole system, replaced the control valve and the terom stat,
what is the solution/cause of this.
many years agao I had a 1980 dodge truck and it had the same problm and the solution was it was airbound cure was squessing the inlet hose(heater core) to force the air out. I tried this with the chevy and got it to work better bt not enough. Is there someting else I should try?