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Try pulling the bleeder completely see if you get flow. I personally would stick with manual its quicker. Have you recently replaced the master cylinder?
What I do is run a clear hose from the bleeder to a bottle with some fluid in it, open the valve slightly, pump the brakes few times, fill the master again, keep repeating until you see no air in the clear tube.
Gravity bleeding will do okay, but doesn't usually get all the air out. Pressing the pedal with some force pulls/pushes all the tiny bits of air out.
your bleeder might be so pluged up from crud either outside or could be inside . turn it / them out all the way and get a small drill bit and turn by fingers till it cleans out the passage way .
To get it flowing open bleeder put m/c lid on and push brake pedal some to get it to started flowing then remover m/c lid and keep it filled. then watch bleeder and close as needed when no more air bubbles. You may have to repeat Remember to replace m/c lid while pushing [B] pedal. Also make sure you adjust all brakes up. This always works for me. orich