Changing a Deore M615 brake caliper over, am I right in thinking it's literally just unscrew cable from old caliper/screw into new caliper/bleed?
Yep
That seems far too easy, something's bound to go wrong - 10 minute jobs never end up being 10 mins!
The mechanical part is literally as you describe it. The hydraulic part as we all know can take months
Yeah, this will probably be where I find out the lever is borked as well and I should just have replaced the whole thing!
What's the problem you're trying to fix?
No power in the caliper- it'll grip, but not very well. Bled it several times and cleaned the rotor and pads with IPA in case they were contaminated but no luck. I'm guessing the pistons are gone.
I've had this with a shimano brake recently, changed the caliper and the lever in the end and still no better. I personally think you can bleed a shimano brake so that it feels like there is no air but even the tiniest amount of bl tiny tiny bubbles ruin their power.
Seems to have worked, changed over and bled no bother, almost feels strange not having the lever pull to the bars!