Harold Kestenholz
Your work with the system and radiator indicates that water can get into the radiator, but not back out to return to the boiler.
There is always the story that is remenbered by a contractor when he repairs something in an old house. The customer says a few days later that the radiator worked until the contractor fixed parts in the adjoining room. The contractor follows the pipes into the wall and finds they were cut off and plugged so long ago that the skeleton of a mouse is found there alongside a page from a newspaper showing the sinking of the Titanic.
