What was found
MLG DISAGREE AFTER TAKEOFF, SELECTED MLG UP AFTER TAKEOFF & RECEIVED GEAR DISAGREE WARNING EICAS MSG & AURAL ALERT. NLG SHOWED UP & MAINS DID NOT. RAN QRH EMER 13-3. MESSAGE PERSISTED AFTER SELECTING HYD 2 & 3B ON. PULLED MANUAL RELEASE HANDLE & SELECTED MLG DOWN IAW QRH. ALL MLG SHOWED DOWN & LOCKED. FINISHED QRH PROCEDURE & RETURNED TO KBED. REMOVED THE MLG SELECTOR VALVE IAW AMM 32-32-10. INSTALLED SERVICABLE MLG SELECTOR VALVE IAW AMM 32- 32-10. FUNCTIONAL TEST OF MLB EXTENSION & RETRACTION SYS WAS GOOD IAW AMM 32-30-00-720-801 NO DEFECTS, LEAK TEST GOOD IAW AMM 32-32-10-400-801. NO DEFECTS NOTED.