How full was the radiator? you're supposed to be able to see the tops of the fins I thnk, so I suspect you over filled it. I've now put an expansion bottle into my system, I started by just putting a bottle on the end of the pipe you're talking about, it pushed water out & then as it cooled pulled it back in. Was going to leave it like this, but as I then put a heater matrix in I've put a proper tank on it from a Rover 100 (I think)
Alan...