wrote

Hi all,

I added a radiator to my sister's central heating system and fitted a
snip.........

Other responders assume this is a pressurised system - can't see that in the
original post.
But, in any case.......
Have you switched any zone valves in the system to the manual position ?
There may be one three-way valve OR maybe one or more two-way valves.
Moving these to the manually open position greatly improves the drain/fill
operation.

Phil