I guess they could leak?
Mine doesn't, but it is really eager to lock up and drive the fan an awful lot. I'm wondering if this is a trait of cheaper viscous couplings - both the Britpart one I put on my first P38 and whatever unit is on my new one do the same thing. It drowns out the V8 completely :(
Firstly, the engine isn't overheating, building any excessive pressure or really doing anything out of the ordinary (then again, it is a Rover V8, maybe that isn't normal). It can sit for a couple of hours at "70" and behave itself, and I can sit in traffic for ages barely moving without any issue.
My commute is say 20 miles on the motorway and then 8 miles or so minor roads through town etc. As soon as I come off the motorway, the fan will lock up, and then never shut up at low speeds. I get that is sort of the point - but its 5c outside, I wouldn't have thought it would take long to appease things and let go again. It will do the same from cold if I'm driving around town for long enough.
The red P38 we have doesn't do this... the fan will lock up from the heat soak in the engine bay if you stop and restart it a little while later, and then cool off fairly quickly and go quiet again. Otherwise it remains quiet - even after a run on the motorway.
What's your experience?