Why is the top radiator hose hot and the bottom is cool you just changed a leaking water pump?

Ok. I am having this problem too. I was told that I need to replace the thermostat. If you have already done this then there is one other thing that I can think of to do. On the radiator there should be some kind of release for air. This needs to be opened when you fill the radiator. If you need more info you can email me at snipermcguns@hotmail.com