Partially draining a sealed system

[HenryC]

I need to find a very slow leak in my sealed central heating system and, to narrow down the search, I want to install a pair of full bore isolation valves in the flow and return pipes that supply the upstairs radiators. Since these valves have compression connections I don’t need the pipes to be absolutely dry. Ideally, to save time and corrosion inhibitor, I would like to drain the pipes (where the valves will be fitted) without draining all the radiators. I was thinking along the following lines: (1) release the pressure by bleeding off some water (2) close the valves on all radiators except the smallest one upstairs (3) remove the top plug from this small radiator (4) drain the system (i.e. the pipework and small radiator) until water level is below where the valves will be fitted (5) fit the valves (6) top up inhibitor by adding via the small radiator (7) refit the radiator plug (8) open all the radiator valves (9) fill and bleed the system. Do you think this would work, and do I also need to close the cap on the auto air vent in the boiler?

 

[gas112]

Close the auto airvent release the pressure and it should vacum up without having to drain , just like the old trick of having a milk bottle full of water upside down.
You can check its vacumed up by removing a rad valve then replacing it before cutting

 

[HenryC]

Close the auto airvent release the pressure and it should vacum up without having to drain , just like the old trick of having a milk bottle full of water upside down.
You can check its vacumed up by removing a rad valve then replacing it before cutting

Thanks for your advice - I will try that when I do the job in the next few weeks. I assume it would still be best to close all the radiator valves first, so that if the vacuum somehow got broken, at least only the water in the pipes would get dumped on my kitchen floor.