The brakes on that car are not unique to the Alero; probably the same basic system (common parts) used on many same-year (and era) GM cars.
So, normal diagnostic procedures apply.
It may be that the booster is not working properly. The main thing I'd be concerned about, though, is whether the ABS system (and the ABS pump in particular) is in good order. How does the fluid in the master cylinder reservoir look? If it's dark brown I'd be worried about whether it was flushed/replaced "per the book." If it wasn't you may be facing some expensive brake work....