the brakes have me stumped

[Sawyer's Datsun]

i have no pressure in my pedal. i swapped the back shoes and the front calipers and pads. afterwards i bled the system only to find out that the master cylinder was bad. i put a new master in and bled the system once again. i have no pressure in my pedal. when i pump the pedal i get stiffness like it should have but once i sit for a second i lose all pressure. my back brakes engage and even lock up but there is nothing in the front. can i get some help?

[fast720]

Hi
Did you loose all your brake fluid for the front brakes ? some brake systems have a shuttle that closes the fluid from front to rear only or the other way depending on the problem, now I am not sure about the nissan but some you can force the shuttle back and some brands you have to buy a new one, the other thing to try is bleed the master first on its own,di you do this ?.
They call it bench bleeding, you bleed the master in a vice and have two hoses going from the master back into the masters reservoir ,you do this to get the air out before you pump all that air into the rest of your system.
I hope this helps
Phil