HISTORY OF FLIGHT: An Embraer EMB-145LR, registration N12922, operated by ExpressJet, experienced several uncommanded swerve events during multiple landing roll outs between the dates of 3/19/2013 and 3/29/2013. The uncommanded swerves were described as the aircraft pulling to the left. The crew was able to regain and maintain runway heading during each event. INJURIES: Passenger counts were not provided by ExpressJet. No injuries were reported. DAMAGE TO AIRPLANE: None AIRCRAFT INFORMATION: The airplane was identified as an Embraer EMB-145LR, registration N12922, serial number 145338. FLIGHT RECORDERS: The flight data recorder was downloaded after an uncommanded swerve event on March 28, 2013, at about 0910 eastern daylight time, during the landing roll at Jacksonville International Airport (KJAX), Jacksonville, Florida. The flight was identified as flight number 4618 and originated from Chicago O'Hare International Airport. The crew reported an uncommanded pull to the left but they were able to regain and maintain heading during the event. TEST AND RESEARCH: The aircraft experienced several uncommanded swerve events during multiple landings between the dates of 3/19/2013 and 3/29/2013. The swerve events could not be duplicated by maintenance. Troubleshooting during this timeframe resulted in the replacement of multiple nose wheel steering system components. On March 29th the nose wheel steering manifold assembly (P/N 308570-1007, S/N 0445B) was removed. After the removal there were no further reports of uncommanded swerving or pulling to the left on the aircraft. The manifold assembly was installed on the incident aircraft on March 8, 2013. A review of the flights from the time the manifold assembly was installed to the time it was removed revealed that the swerving events were reported on the longer duration flights. The manifold assembly was sent to Parker Aerospace (the component manufacturer) for component level testing that included cold soaking to simulate the conditions experienced during longer flights. The nose wheel steering hydraulic manifold assembly was examined at the Parker Aerospace facility in Irvine, California on June 3-4, 2013. Testing of the manifold assembly revealed that there were unexpected pressures observed at the output chambers after the unit was cold soaked. The electrohydraulic servo valve (EHSV) is a sub-component of the manifold assembly and was removed for further investigation. Testing of the EHSV occurred at the Woodward HRT (the component manufacturer) facility in Santa Clarita, California on June 5-6, 2013. The pressure anomalies observed during the manifold assembly tests were also identified during this testing of the EHSV. A teardown and examination of the EHSV identified the presence of a clear gel-like substance between the armature and the coil. Laboratory analysis revealed that the substance was similar to a thermally degraded MCS-352B lubricant. Pressure gain plots showed that hysteresis improved following the removal of the substance from the armature. No other anomalies were identified during the examination or teardown of the EHSV. The EHSV manufacturer indicated that there should not have been any substance in that area and that it was unknown how the substance could have gotten there.