I have found that when installing a new or rebuilt M/C you need to bleed just the M/C first.
With the M/C mounted, leave the brake line unhooked. Place your finger tip over the hole the banjo bolt threads into, slowly pump lever. As you pull the lever air gets pushed out around your finger tip. As you release the lever your finger tip seals the hole, this makes the fluid get drawn out of the resevour. After a few pumps you should get fluid spraying out past your finger tip.
Now hook the line up. It should bleed ok then.
I have an extra set of bars I use for this. I do this bleeding off the bike. I also bleed the line the same way Hook the line up, hold the line up above the M/C, seal the end with my finger and thumb, bleed, once I get fluid I hook the line to the next thing in the system, With a two line system I hook the line to the junction and the next line, hold up, bleed. Hook the line to the caliper, hold up bleed.
Once all bled I install the assembled system on the bike.
A one piece line is quicker.
Leo