Very little heat
 

Here's the issue. After driving my car enough to let it heat up, I can turn the heat on and get a little bit of heat out. The temperature of the engine is fine. After a short while the heat cools down. I was told I might possibly have Air getting into my system. When I shut the car off I do hear what sounds like air being sucked in from somewhere and I can't find it. There also seems to be no pressure in the system. Even when the car is on and hot I can take off the cap from the overflow tank and there is no pressure. I have flushed out the heater core and that is fine. There is a hose down by the driver side of the block that splits off with one end going to the overflow tank and the other and going to the engine. The hose going to the engine seems to be very cool. I have no coolant mixing with the oil so I'm pretty sure it's not a head gasket and the thermostat is working fine. I just can't figure this out. And with the cold weather almost here I really need to figure this out. Any suggestions?