21psi is too high, here is a couple of things I would be doing first.
Using a mitty vac at the acuator, and while obseving the position sensor apply full vac,ensure the rod is fully pulled on and check the sensor reading. Then let it release and veryfy the rod has fully returned, even try to push it back with a screwdriver incase it's sticking, and verify the sensor reading.
If thats ok, I would then check vac at the actuator at idle, there should be enough to fully pull on the rod ready to produce full pressure when driving off.
Next bit is a bit tricky on the road, but you need to confirm the vac at the actuator is being reduced as boost rises under full load.
Sounds to me like the vanes are probably sticking on this one, but if you do the tests above it will help us to help you.
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