Clausing Variable Speed Drive

So I had some vibration in the headstock of the Clausing 5913 and wanted to get rid of it.  Also, the variable speed didn't work at speeds under about 600 rpm;  it would wander down to the minimum speed. So, we took the drive unit apart, replaced the belts with new ones, inspected all the components, and bled the hydraulics.  Now it operates as it should, but I still have a vibration coming when the clutch is engaged.  Possibly coming from the upper belt. I can work with this as it is, but would appreciate any suggestions on how to reduce the vibration associated with the clutch/upper drive, More info, including the alternative bleed method for the VSD hydraulics is in the files section here; https://groups.io/g/clausingusers/mes...