When the brake fluid is replaced, make sure it is done with a pressure bleeder with around 20 psi pressure. There are some places in the circuit from which the air is difficult to remove, and pumping the brake pedal or vacuum bleeding will not get those bubbles out.
Once you are certain that there is no air in the system, the next suspect is a hose that is bulging at one of the wheels.
Another problem that causes a symptom similar to air is a bad wheel bearing that is letting one of the rotors move a lot and press the caliper piston well back into its bore. When you apply the brakes, that piston has to go a lot farther than the other three, resulting in soft brake pedal and uneven braking. You can check for that by jacking up each corner of the car and grabbing the top of the tire and jerking in and out to see if there is any lateral movement of the wheel.