What's a good way to fill up a system pipe on drops.

[WaterTight]

So every rad is fed from drop coming down from loft, some just feeding that rad's flow and return, one or two teeing off to another rad too.

Thanks

 

[Best]

I assume you mean how to avoid air trapped in middle of pipes?
Not a lot you can do other than just bleed the rads on the drops and then finally open any manual vents in loft (if fitted).
If you did it the other way - open vents in loft first (or have auto vents working, which I personally avoid due to them leaking) you will still risk get air trapped in any horizontal pipe runs at rads. No big deal though as just turn other rads off when heating going usually shifts air I find.
If you are piping the job new, I think a bit of care to keep any of the horizontal runs level, with no high spots and have the main runs of pipes in loft supported and with a slight rise to tee offs with manual vent points, should ensure no bother.

 

[hammers4spanner]

Any issues I have had in the past with circuits like this I have just isolates the rad or rads with probs. Pressurised system to about 2.5 bar ptfe rad tail into bleed cap , put hose on the open up F and R individual. Pressure blasts air straight out.