2000 Chevy Silverado, cooling issue.

I own a 2000 chevy 5.3.I recently repaired the waterpump, and the head gaskets bc of leaks. Everything was great for about two weeks, now my problem is that once the truck has been running for a while, once it gets turned off, shortly after the water gushes out from the radiator reservoir tank, which is located on the passanger fender, it gushes out through this hose that is along the side, it doesnt connect to anything, I guess its supposed to relieve air perhaps. So my question is. Do I need a new radiator or is there an air bubble of some sort. If so how do I remove the "air bubble"

thanks