A dump Valve releases the pressurised air in the intercooler and pipework, when the throttle plate is closed down after any +ve boost running
An internal (or external) wastegate relieves exhaust gas away from the turbo exhaust wheel, to regulate boost levels.
All OE Subaru turbos have an internal gate and an actuator, turbos without an internal gate setup also require a mechanism to regulate boost - An external gate that attaches to the exhaust manifold/uppipe.
Try replacing your Bailey DV with the original item to test it
