Brake Bleed

In desperate need of ideas.
Have rebuilt braked all round and now changed master cylinder too.
It's the non-servo version.
Having bled the brakes I managed to get brakes but they have never stayed.
I then changed the master cylinder, but I now have cannot get nay brakes at all.
When I release the bleed nipples there is no fluid there at all - not a drop. the fluid just stays in the reservoir when I try and bleed them. It seems like I have some sort of air lock.
Any suggestions please???
Thank
Marc

slacken pipe coming out of master cylinder ,then press pedal,tighten before letting pedal up repeat till fluid and air are expelled then bleed normally at wheel cylinders