How can I get the brake tubes inside the frame?
On my kid's bike, we installed the XT M8000 set too.
The hose and the lever didn't come assembled totally.
It was easy to unmount and there was even a plug preventing the oil to leak.
However the plug had to be removed from the hose to pass it through the hole on the back of the frame.
Then cut the hose to the right length, turning the bar to the maximum right( as many times on a bike, measure twice, cut once ).
And then assemble the hose with the lever.
Beware to not forget to put the screw first on the hose before inserting it into the lever.
I forgot
and some oil leaked when removing it, for a second try. It works ok though, but I will need to bleed it one of these days.