Assuming you have the top port vented then? With your current configuration the MBC is bleeding off the pressure that would be acting on the diaphragm to open the valve. If you switch to the top port, then the MBC would be bleeding off pressure that would be acting on the the diaphragm to hold the valve closed. You need to be careful with this config, though, as you will run a LOT more boost, so start slow and make sure you've got more than sufficient octane in case of an overboost.
Another option is to T the pressure source off of the compressor to IN side of boost controller and the bottom/side port, and run the OUT side of your boost controller to the top port -- this is how my EBC solenoid is run.