Getting no fluid out of a bleed nipple
Look at the simple things first, is the master cylinder topped up with fluid? It is surprisingly easy to forget to do this! If you have fluid in the master cylinder, try removing the bleed nipple entirely and see if you get flow under pedal pressure.
Case 1. Fluid squirts from line: You have a blocked or corroded bleed nipple, which will need to be cleaned out with an awl or similar pointy tool, or replaced.
Case 2. Still no fluid: Unscrew the hose at the car side so it doesn't twist or kink, and see if you get a stream of fluid under pedal pressure. When testing, have a wadded rag over the end of line to catch the fluid - the pressure is great enough to spray the other side of the garage, get in your eye or on your paint, etc. Also, press on the pedal about a dozen times when doing this; if the fluid all drained out of the system before you started bleeding the brakes, (if you overhauled some part of the brake system) it can take a surprisingly long time to pump fluid throughout the system.
Soft brakes even after bleeding
You probably still have air in the system somewhere. If you have had parts of the brake system disconnected, ensure that the bleed nipples for the wheel cylinders and front callipers are at the top, so that the air can all escape during the bleeding process. (The air bubbles rise to the top.)
Front brakes bled, but no luck at the other end
The pressure limiting valve may be operating, try to press slowly on the brake pedal so that the fluid goes through with out making the valve close of, or the pressure limiting valve is seized this also means that the back brakes don’t work either. If so the valve will have to be repaired.
You have tried all of the above, but still have no luck!
You may want to invest in a simple vacuum pump to bleed the brakes. One such tool, is the "Mityvac Vacuum Pump / Brake Bleeding Kit." The pump is applied to a bleed nipple, and pulls fluid through the system. This technique has the advantage that the master cylinder does not require "priming" in order to get the bleeding process started.
In my experience, the vacuum pump does a good job and is a one-person operation (with the exception of checking the results in the "conventional" fashion).
If you haven’t fixed your brakes what’s stopping you?