Brake bias?

Lift the back of the car up with a floor jack and place the trans in gear to make sure the back brakes are stopping the back wheels. I have seen wheel cylinders go bad were they leak air back in will not work--you bleed them they work for a while then stop working but do not leak brake fluid.

Just make sure both back wheels are high enough from the ground and always be sure to shut off the motor if the car starts to move or if the back brakes are not working shut the motor off, don't place the trans in to park with the back wheels spinning.

If the back brakes are working, not sure what more can be done.