Is it coming out with the cap on? Coolant expands with heat so you need to fill the main chamber as full as you can. Install the cap and then fill the overflow tank. IF the overflow tank gains fluid then the first thing I would replace would be the pressure cap on the the chamber (they can lose their seal and or spring tension). If that continues to gain fluid then you can have the fluid tested to check for combustion gasses or simply get ready for a head gasket replacement.