I think Larry want you to replace the rubber diaphragm in the vacuum pump. You need to repair the vaccum pump as soon as possible if it is the problem. A leaking rubber diaphragm in the vacuum pump can cause other damages (search this forum for details). The later 300D does not use diaphragm type pump. The vacumm pump problem should not cause the brake to lock up (one or more tires do not turn causing the car to skid).

David