Problem: You try to gravity bleed some brakes, but nothing comes out when you open the bleeder.
Solution: A quick way to bleed brakes when you're by yourself is to use a vacuum check valve. Most techs probably have a few of these laying around in the misc. parts bin. If not, these valves are commonly found near vacuum supply lines that run to reservoirs and climate control systems. The one shown in the photo below has three ports (it's all we had for a picture of a check valve) but you could block off that extra port if that's all you had.
Typical vacuum check valve found on Chevy Astro van
Attach some vacuum hose to each end of the check valve. Attach the end you can blow through to a slightly cracked bleeder. Place the other end in a catch container. Then pump away.
Here's a few links to complete articles on brake bleeding...