A B737 pilot conducting preflight for a maintenance check flight reported they discovered standby rudder system A and B were not functioning.

Narrative

Standby rudder system A and B both failed the flight control check during a preflight for flight control confidence/manual reversion Functional Check Flight (FCF). I believe maintenance did not check the function of this system prior to calling the aircraft crew ready. A failed standby rudder system in flight could result in loss of controlled flight if both hydraulic systems fail. If an engine failure was combined with a complete loss of hydraulics and no standby hydraulic system the an immediate loss of control flight would occur and the plane would be lost. This is second time in 4 years I have found a failed standby hydraulic system.I would suggest having maintenance perform a ground flight control check more routinely when planes exit maintenance. The ground flight control check takes 7-8 minutes to perform; but checks the normal and standby functions of the flight controls. I consider the standby rudder system to be one of the more important back up systems we have on the 737 fleet and should be routinely checked. Only because of an elevator that was removed and reinstalled did we perform an FCF. Had the elevator not been repaired; this plane would have been flying around with a bad standby rudder system indefinitely.