Definatly not the booster itself.
Could be a defective Master cylinder.
The light coming on indicates the pressure in one end is greater than the other.
Rear brakes adjustment? Have you looked at them? It could be the adjustment is too loose.
One way to tell - pull up on the parking brake handle to find out where it starts to get tight. Now loosen it slightly.
Drive it - the first time you press the pedal it will be as before, but subsequent pushes it'll feel normal.
Putting the E brake on slightly tightens the shoes - if they were too loose, it will feel like it should. If it doesn't help, suspect air in the lines, either front or rear.
Most folks do not bleed the air out properly.
Let us know, we can guide you trough the RIGHT WAY to adjust the rears, or the RIGHT WAY to bleed all the air out, depending.