How long was the car in storage?
Sounds as if you have air in your brake system or a bad master cylinder. Both could be the result of the deterioration of rubber parts in the system.
If it was my car, I would drain all of the old brake fluid and bleed the system with fresh fluid. If that works, monitor the area under the car for brake fluid leaks, especially around each tire.
If bleeding the brakes doesn't work, it is likely a master cylinder. Rebuild or replace it.
I have little experience with ABS. However, it sure sounds like one of these two problems.
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1966 122s, 1968 142s, 1969 144s, 1979 245dl, 1989 244gl