FYI
The lines can leak air internally, and show no visible signs of air or fluid on the exterior.
It is rare but I have personally seen it twice. Once on a Gsxr1k rear brake line...and once on my B-king clutch line.
Plenty of pressure at the rebuilt master cylinder, and pressure at the slave cylinder with a new seal.
Fluid vacuumed down the line, and still no pressure.
Multiple attempts, all no good.
I ordered a new clutch line, to eliminate it as the issue, beliving it was sucking air somewhere inside, as the lines are multi-layered, but still not beliving it could happen without me being able to find an exterior leak.
I put the new line on and had lever pressure within seconds of vacuuming fluid through it.
I was suprised, but it worked.
Same thing with the rear brake line.
And no, it would not be uncommon to fail when disassembled, as the pressure has been relived, but once applied again, sometimes that's when old parts have had enough.
And
If you're not drawing fluid through the line with a vacuum, or pushing it up the line with a syringe, you will most always have to wait a long time to gravity bleed motorcycles brakes/hydraulic clutch lines, if they gravity bleed at all, as some are just stubborn and will not.
Vacuuming the fluid through is the easiest way.