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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?

No, I said I thought you had several issues. The light comes on only when you have a pressure difference front to rear. This can be a leak or air (air is compressable )in the lines. When you hit the pedal again and the light goes off it's probably due to the fact that you built up enough pressure from the first application to center the valve and turn out the light. If no leaks, most likely you still have air trapped in a line somewhere. If the grabbing occurs after the car has sat for a week or more ( depending on humidity, etc ) you could be right. In the fleet business it was refered to a "lot rot" - where drums / rotors / shoes / pads would rust up. The "firm pedal but poor stopping action" is another problem. Could be a booster issue - or - If a car sits a lot, sometimes the shoes can get rust inpregnated from continually cleaning off the drums. Next thing that happens is they glaze and then they don't "bit" the drums very good, especially after they heat up. You might want to pull the drums and try sanding down the shoes with 80 grit paper and see if that helps.

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