Test the actuator with a mityvac hand pump, not by disconnecting line and forcing overboost.
With car off pump up the actuator and watch the arm move. Check what pressure it takes to move. If it pushes right open at 1-2psi, then you have a broken spring. They do break.
If your diaphragm was broken pressure wouldn't move the arm, or it would take much more to move and obviously it wont hold pressure. But that would cause higher than normal boost, just like a bleed.
Others say your arm is adjustable. If so tension so it takes about 10psi or so to get the actuator arm to move when pumping up with the hand pump.
Wayne H.
'91 Dodge Spirit ES 2.5L turbo 5spd
'05 PT GT 2.4T HO autostick (RIP)
'89 Plymouth Acclaim 2.5L turbo auto, "Slugmobile" yes, THE Slugmobile!
'89 Dodge Caravan SE 2.5L turbo auto, "Mean Mini" yes, Gus' Mean Mini! (Current best 11.699 @ 114.43 mph! - Oct 15th, 2022 Cecil County Dragway, MD)
MeanMini dragracing videos: https://www.youtube.com/playlist?lis...URZLB1RxGYF6vw
and other cars, trucks and motorcycles
https://www.youtube.com/user/SlugmobileMeanMini
I've found that on my Spirit R/T's (stock turbo), if you route the waste gate line directly to manifold source (no controller), I usually see about 6-7 psi at WOT minimum. It might go a little higher at lower elevations than where I'm at though.
In addition to testing with a Mityvac, an easy check would be to hook up the waste gate can directly to the intake manifold and see what your minimum WOT boost is.