Draining down 'dodgy' systems is always a nightmare, it can often take longer to drain down and refill than the actual job you were there to do!
That's why we use the large Co2 cylinder and freeze the pipes wherever we can.
This week I had to break into a one-pipe system to extend it. Two freeze clips on, within 4 minutes both sections were solid, cut out the middle bit of pipe, valve off each end (for later connection of new rads), fit new section of pipe and all done.
The whole job took about 20 minutes!
It would have taken at least that to drain down!
And as for refilling, well that could have taken hours with all the rads to bleed. That's assuming there are no problems................(see Windy Miller post above!).
I also save the cost on renewing the inhibitor
Aaahhhh, technology is a wonderful thing