Dodge Repair Questions? Ask a Mechanic for Answers ASAP
It sounds like you may have an air pocket in the system, or the radiator cap is not sealing and allowing the coolant to coil off. You may want to drain the radiator and capture as much coolant as possible to not waste it. Then jack up the front of the van so that the highest point of the system is the radiator fill hole. REplace the cap if you haven't already with the new radiator.
Another method to filling the system without getting air pockets is to use a vacuum filling device like this one here: http://www.amazon.com/Radiator-Cooling-System-Vacuum-Refill/dp/B0081N522E
You may want to have the system block tested for a blown head gasket.
Is there anything else I can help with?