Beech 1900
Flight Crew Commercial
Confusion; Troubleshooting; Training / Qualification
Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Maintenance; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Became Reoriented; Flight Crew Returned To Departure Airport; Flight Crew FLC complied w / Automation / Advisory
Procedure; Human Factors; Chart Or Publication
My First Officer and I accepted a plane that had been used by Maintenance earlier that day to perform the first flight of the day run up. We did not realize it at the time but Maintenance had left the Dump Valve ON from their run up. As it is not part of the run up SOP to leave the Dump Valve ON; we were later confused at it being ON. As it is not part of any Flows; Checklists; or even Emergency procedures (for cabin decompression) to check the valve; I did not check it until it sparked in my brain on final approach.  By that time we had already called air return to Dispatch and did not think it wise to cancel my landing and just continue on. There should be at least one checklist/ flow checking the dump valve. The emergency checklist for cabin decompression should reference it.
[Report narrative contained no additional information.]
During the B1900 maintenance first flight run up the Dump Valve was mistakenly left ON. After takeoff when the cabin would not pressurize; the flight crew returned to the departure airport and just before landing realized the Valve was ON but its position is not referenced in any checklist or flow.