with the system up and running feel the pipes that supply the rad one should be hotter than other (feed) make sure the lock shield valve is fully open on the hot pipe end, then open other end of the rad (return) by removing the dummy plastic cap and with a small adjustable spanner,open 1-1/2 turns from fully closed position now with apiece of rag held under the bleed valve bleed the rad free of any trapped air till hissing stops and water flows out close valve rad at this point should now be getting HOT.
If your boiler is a pressurized system you will have to add more water via filler loop if the pressure HAS gone below 1 bar working pressure
wirecutterman, March 2008