1999 GMC Sierra Heating Problems

Since the weather turned cold, when I call for heat in the cab of the truck I only get cold air. I have checked the hoses going to the heater Core and the supply hose is warm and the return hose is not. Using an air compressor I added some pressure to the trucks radiator system which resulted in a little more heat but not up to normal. Any Idea if there is water control valve at the heater core that may be not functioning or if the core needs to be flushed.

I have also thought that maybe there is an air pocket traped in the core.