Sometimes if it is overfilled, it will dump it out till it finds the correct level. Mine will dump it out if the level is a bit over half way in the plastic coolant tank. If you just got a new radiator, I doubt if it would be hoses - but ya never know.
Fill the reservoir up to half way and see if it dumps it again. It is preferable to use an antifreeze mix, as water will dilute things a bit.
Also go for a drive with the heater on full blast, there may be air in the system somewhere.