1992 300SE. Checked DTC codes from PSE system and repaired leak in trunk handle/close assist circuit. Cleared codes.

Switch key to "on" - pump runs for about 3 seconds, cuts off for about 2 seconds and repeats this cycle about 20 or 30 times - then is silent.

Start motor - process begins again. Don't know if it continues or ceases after same cycle as with key on, but I think it does stop at some point.

Cut motor off, remove key. Cycling of pump begins again and continues until (I guess) it gives up trying to do whatever it is trying to do.

No codes are displayed with a DTC check.

During this time (key on-engine off, or key off-engine off), far as I know, nothing is going on that should require the pump to run.

Things that I know require the PSE pump:
Door locks
Trunk handle/close assist
Reversing Masts
Lumbar Supports (don't have the orthopedic versions)
Rear headrests (have the ones that release manually but have to be reset by hand).
Manifold vacuum assist - don't know exactly what this means - maybe something to do with HVAC when engine isn't running?

Everything seems to work as advertised except for the lumbar supports - nothing happens regardless of position of dials, so I've set them both to 0.

Is this normal behavior? Note that while I'm doing all of the "de-gremlining" work, the back seat is out and the pump is fully exposed - that's why the cycling is so noticeable/annoying.

I would suspect that the vacuum assist process would charge an accumulator somewhere and only have to run when vacuum is required. I'm thinking about disconnecting the manifold from the pump and trying to figure out which circuit is the active one when all of this annoying cycling is going on.

Any suggestions would be appreciated.