I recently had the problem that my Standard S2 was only boosting 3 psi.

Then I checked all the common troublemakers

- actuator (length and spring)
- amal valve
- searched for air leaks
- etc.

But finally it turned out to be a faulty ECU. After changing it, it boosted 7 - 8 PSI again. Maybe the vacuum sensor in there was knackered.