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While you are at it, replace your brake lines, (chassis to caliper, 4 of them). You are riding around on 25 year old rubber lines. They are cheap insurance.
Any "system" is only as strong as the weakest link....."olde chinse proverb me thinks"
I have the same problem on my 87. I replaced the booster with one of the new "plastic" ones and it isn't any better. I also replaced all the old rubber brake lines with new SS lines.
I am starting to think it is a vacuum problem with the booster not getting enough vacuum and therefore you must force the pedal. It is only a guess and right now my engine is out of the car undergoing a mild rebuild.
Best of luck, as for my problem I will continue chasing it once I have a motor in my car again.