Braking woes

This year I replaced the rear brake cylinders. I also got a rebuilt master cylinder from a friend so that went on too. I bled the right rear first and then the left rear...no problems so far. Then it came to the front, tried the r. front no go, then tried the left front, no go next day I tried inhaling via hose and got left front to work, tried right front and never got anything. What should i try next. Oh yeah went from DOT 3 to DOT 4 also.
What should I try next???