[JFP in PA]

Clutches fail of all kinds of reasons and at lots of different mileage levels. After sitting for 15 years, just about anything could be wrong with it, we have seen problems with low mileage clutches on "barn find" vehicles that sat for decades, so it is nothing unusual. You could simply have some corrosion built up that is keeping it from correctly engaging, but driving it around and letting it slip is not doing it any favors. If bleeding the system has not improved anything, it is time (perhaps past time) to take it apart and see what is wrong. It is not going to heal on its own..........