Thanks ku95. I checked: the reservoir is damp and dirty underneath, and perhaps crucially, the level is at minimum. Also, I can see "wet" spots in a descending line of tubes underneath it, possibly where it drips or the serpentine belt throws it. But there's nothing apparently wrong with the connection between reservoir and pump. Certainly not with the main front seal either. I will change the o-ring anyway. Special request: Can you point me to where you got the part, or the part number?