Leaking inner dual tire, need help finding leak

I would ask them to tank test the tire/wheel/valve assembly at the normal pressure you run and it leaks at. Inflating over normal max can hide a bead or valve stem to rim leak by seating the offending area, or cause the tire to seal around a small puncture object, maybe. 8 psi in a day is a really slow leak - may be a 2 coffee detection time...