[Closed] Trimming hydraulic hose without draining/rebleeding

When I got my first set of hydraulic disc brakes a couple of years back I'd heard that it was possible to trim the hose and refit it and, if you were lucky, you wouldn't get air in the system. So that's what I did, and it worked.

As I've learnt a bit more about the Shimano setup and done some bleeding and fitted new calipers I've realised that the port is open when the lever is in the not depressed position (when bleeding, with the bleed nipple open fluid will sometimes flow out from the resevoir without touching the lever. So when I removed the shroud nut from the lever and pulled the hose out was I just lucky not to lose my fluid?

I've got a new set of Shimano brakes to fit to a small frame, so the rear hose at least is likely to need a trim. I'm perfectly capable of draining and rebleeding the sytstem, it's just that I'd rather not have to if I can get away with it. Can I remove the hose from the lever of a primed system, trim it, replace the olive/insert and refit or should I be draining it first. I probably haven't quite got my head fully around how hydraulic systems work.