On our 78, the slider was also harder to close than I remembered our old 84 being. I did some digging around and found that the lower slider rail had a bunch of grit/dirt in there. I clean it all out and then re-lubed the slider rail. I did the same for the top rail.
The rear "hinge" piece was already cleaned in our case as I had just replaced it with a junkyard one. Before I installed it I cleaned the heck out of it, the lubed it.
Now the door slides easily, and closes just about every time, first try, never needing to be slammed.
~Mark