[hvychve]

Nothing wrong with tracing your steps, I would verify your wife is holding firm on brake pedal while you crack bleeder, if she is doing so, than I would verify you have good adjustment on the rear brakes, if adjusted good and you bled entire system including master and have the correct rod from master to booster, you should have good pedal. a bad booster will normaly just give you no assist when running. what sounds like you have is air in the system, sorry, I know you said you have no air, but sure sounds like it.
try gravity bleeding for a while, open the right rear bleeder and let the fluid push out any air, leave it open for a while, than close it and do the other rear, than do rt. front and finaly lt rear, making sure you keep resevoir full of fluid, if it goes dry, well, you have to do it all over again. than after gravity bleeding you can have her get her strong legs up there and hold the brake pedal down while you crack bleeder , noting if any air or bubbles come out, do each wheel a number of times, if you have air up towards the master , you will have to bleed several times to push out if its in the rear lines.......

now if its not air, like you say you are pretty sure it isnt, and probably mad by now, due to fact I say it sounds like it is.........

are your calipers on the correct side, bleeders should be on the very top , so air can be removed entirely, if they are switched from side to side they will have the bleeders too low to escape all the air..

No other leaks , like rear rubber hose or any of the front hoses?

well, I cant think of much more, but good luck and keep us all informed.