Just noticed your thread. One thing you might check if you have not taken it apart yet -- make sure that the knurled part of the speedometer cable is not loose (where it seems to actually be leaking). I had a leak there, and it just needed to be tightened a little. Of course, my '79 RS only has 275,000 miles on it.

 

Thanks Blue79RS. It seems tightened (it rotates 1/8 turn left / right with the actual part I tightened it to, stopped by the other part, but I guess that is normal?)