Thanks Crizila! I'll try to explain a little better. The issue with the brakes grabbing really hard is mostly gone at the time I get the car out of the driveway. No warning light when this happens. I think it only might be a case of dry parts binding a little as the car motly has been garaged the last years.
The other more serious problem usually comes after a few miles of driving. I hit the brakes and the pedal gets firm but the brakes don't grab very much and the warning light comes on. After releasing the pedal and hitting it again the light goes off and the brakes work like normal again and usually do the rest of the trip. You say that the light only indicates air or a leak in the lines/system. Then we can stop looking at the booster, right?