John'* got you covered on this one.
If you pull that vac line off the bottom of the actuator, the PCM can't command the BCA to dump boost via the BCS.
In a nutshell, the boost control SOLONOID controls the ACTUATOR. Lots of factors are involved in what and when and why the PCM tells the solonoid to do something with the actuator. The actuator is DUMB, and only does what it'* told by the solonoid. So if your solonoid is bad, you can prevent it from telling the actuator to do the wrong thing by disconnecting that line (don't leave it that way as a fix, as it could be a bad thing later).
Disconnect that line, if the boost returns to normal, either fix a vacuum leak or replace the solonoid.
If the line being disconnect makes no difference, your problem is most likely a vacuum leak or a faulty actuator.
Or as John mentioned, a leaky SC outlet gasket.