Truck Heater Not Working?

I have a '99 e250 truck. The heater core hoses won't get hot neither will heater core. I replaced all hoses and radiator including thermostat. After a fix for overheating, neither the heat core lines nor the core will get hot. Stop Leak has been put in old radiator. I used a hose to flush the core and did get water to go through under high pressure, from hose.

By Erick from Baltimore, MD