B757-200
Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Less Severe
Flight Crew Landed As Precaution; General Maintenance Action
Aircraft
During flight test of [a] B757; cruise altitude FL380; we saw the flap gauge flicker between 0 and 1. Did it again but stayed in the intermediate position followed by a Leading Edge Slat disagree light and EICAS message. Slowed airplane down and got a descent to FL190 to figure out the problem at 210kts to be below any flap limitations. There were no roll or unusual flight control issues. Ran and complied with the QRH checklist; and since it was a flight control/nonstandard approach procedure [we advised ATC] and returned to [departure airport]. No issues or hot brakes during or after landing. Had Fire Department check brakes; they followed us in with no problems.Maintenance thinks it is a malfunction of a relay in the leading edge system. Number 2 FCU (Flap Control Unit) showed faults in system; I don't have all of the details.
B757 Captain reported a leading edge slat disagree light during cruise on a test flight.