When I bleed brakes, I don’t open the bleeder screw very far. Only about a 1/4 turn. If they’re turned ½ turn or more, it can allow air to re-enter the system past the threads.

 

I had a problem once with bleeding my brakes, there was a bad gasket on one of the lines and everytime we bled that brake and let of the brake pedal, air would get pulled back in to the lines. almost had to bleed the entire system. check for leaks