Is it possible for the fuel pressure test valve to go bad and slowly let the pressure leak down?

Are you talking about the schrader valve on the fuel rail?? If it was leaking fuel out then yeah.I'm not sure what your asking??
Are you saying you have fuel pressure leaking down when engine is off?Are you using a gauge to determine this?If so then turn key on to get a fuel pressure reading then see if pressure holds.It should not drop for awhile.If it does drop right away then turn key on to get fuel pressure back up then pinch off fuel return line and see if pressure holds. If it does hold now then you have a bad fuel pressure regulater.If pressure still drops then turn key back on to get pressure up once again then pinch off line coming from pump. If pressure now holds you have a bad fuel pump check valve(bad pump).If pressure still drops then you have a leaking injector.As your turning the key on to build pressure you might have to cycle key on and off a few times to get pressure completely up because GM computers only run the pump 2 seconds with key turned on engine not running.And when I say pinch off lines I mean the 'rubber' ones of course!! Hope this helps.