Hey guys. It's been a couple years, but I figured out why the clutch kept slipping on my datsun. As it turns out, there is a small seal inside the throw out bearing housing, on the back of the input shaft. It's supposed to seal the push rod that runs inside the shaft, between the bearing and the pressure plate. I was only able to find it through the dealership; called a "speedometer pinion seal". Odd name if you ask me. I did this at the beginning of the year, and it's been holding up very well. I actually enjoy driving my car again.