Why is my coolant coming out of my exspansion tank?

Ok but I got a kit thats checks if pressure is coming out of the head gasket and its not that. The thing is as the coolant starts circulating it does fine until it comes back to my exspansion tank then that when it starts backing up coolant. Im thinking mabe my radiator is clogged, because the car did stay park under the porch for almost a year. when i flush it out it had a rust color to it and lil pices of debri coming out.