On Friday, 13 September 2019 19:32:57 UTC+1, Roger Mills wrote:

The unused legs would be better filled with water rather than air. How
about connecting each flow and return together through an isolator valve
behind the blanking plate? You could then allow them to fill with water
and then turn off the valves.

I could always bleed the pipes by briefly removing the stop ends. That's one of the beauties of push-fit for me; the ease with which joins can be made/unmade. I've done many a 'live' capping using push-fit stop ends and when the pressure's on (pun intended) you can't beat a toolless fitting.