Yes all it takes is a caliper to be 90% full when pumping fluid which is why I tend to turn them up side down when bleeding to make sure they are full of fluid and not air.
The simplest method is to get a pint glass top up resivoir then long plastic tube over nipple and pump the brakes four times until the resivor needs topping up.
Never bleed it bellow the line so no air gets in.
Its a longer method of bleeding, but two man jobbie can be just a pain in the arse, as he never pumps and lets go at the right times, hence a pint glass and longer tube so only fliud goes back up the nipple and no need to do up the valve nipple.
If you are really having problems I would raise the passenger wheel arch higher than the other four wheels then do the same thing bleed in order, but raise the back up first due to the gravity helping to get more forced fluid in the back!
I've never had to muk about with bias valves unless it was retro fitting different calipers on the back and needed adjusting.
