hmmmm......, well if your user name is how you drive, it will explain the stopping problem x2bord

, but seriously if you've changed the boosters and have the same problem it may mean that the brake lines all the way down to the wheels have some restrictions thereby reducing the flow pressure of the booster, given the age of the car this is understandable and is often caused by foreign objects getting into the reservoir or by the normal wear and tear of the rubbers in the booster, lines & brake pistons. To fix this problem you'll probably have to do a complete clean and flush of the system, speak to your mechanic about it, if he can't help I can refer you to my mechanic.