Seen this topic come up many times before. I even had the same issue. You will more that likely hear a bunch of Hammer Spit as to what the issue is. First i would try a cranking compression test to rule of worn out rings. Next focus on a intake leak and then valve seals. only so many places to...
not sure why this keeps coming up. suck and blow on the cap if air goes both ways it is vented. the spill out the back of the tank is protected by a flap in the neck of the tank. if there is not one well sorry your tank is not up to snuff.
I was so hoping to see a burn out Lol. one more good bleed can't hurt one other trick i have seen was to run a plastic line from the bleeder on the brake cylinder/caliper and run it back up to the master cylinder reservoir then pump the brakes. this way you are sure not to let the reservoir go...