if you dont have the decorating cap , turn the valve off , drain rad into a builders rubble bag (easier to get around pipes and catch the water) then get a 15mm female iron blanking cap and fit it onto the open trv end.
on the heating side the pump pulls heat out and pushes cooler water in therefore causing circulation. on a gravity system the pipes have no choice but to circulate unless airlocked.
fitted it in a ground floor flat living room. it can be a swine to fit if your corners are not square and your walls are not straight.Took two of us a day to fit 16 m.