My HydroVac is a Bendix brand, the site plug/oil fill port is on the end of the large vacuum can, the plug is quite low on the end as it is a deterrent to overfilling. I use motorcycle fork oil, the original specs called for shock absorber oil, I figure the fork oil is about as close to shock oil as I can get. I rebuilt my HydroVac, the kit was complete with a leather vacuum seal, I would expect yours to be the same.
