[Matt B]

Just pulled my plenum today (Marc's DVD is SO cool!) and did a leak down test on the entire secondary vacuum system. Indeed the actuator seems to be leaking. Actuators work perfectly fine and hold vacuum for many minutes. Also all other hoses and connectors are super tight.
Only the check valve was a bit leaky, but when I blew it with some compressed air a tiny piece of debris fell out and now it work perfectly well, too.

So, time to order a new selenoid and hope this will resolve the short hesitation when Secondaries kick in. Thanks for the advice!