do that direct vac line thing mate, should not take much time to changeover....like I said back of around 10Psi.
This test should get you the absolute minum boost your config will run.
If you get up to 10 psi with it setup like this, you have an actuator problem, or potentially a stuck flapper valve (if shes stuck she will just boost to the moon and back)
mines got a 3inch straight thro exhaust, no mufflers cat or anything (sh*tbox racer) and it sees max 4.8 Psi in this same config.
If you hook a vac line from your air compressor, to the actuator and activate it with less than 10psi pressure (use the compressors regulator set right down, regulate it, if you smash like 80psi in there you will bust it up).......you should be able to see the actuator rod move and the flapper open, its ina *beep* of a spot so maybe record it on your phone.