A MECHANIC AND INSPECTOR REPORT ABOUT A REQUIRED PITOT/STATIC SYSTEM LEAK CHECK THAT WAS NOT PERFORMED; AFTER AN AIR TURNBACK OF A B737-800 AIRCRAFT; DUE TO LOSING CAPTAIN'S AIRSPEED AND ALTIMETER.

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ACFT XXX AIR RETURNED DUE TO LOSING CAPT'S AIRSPD AND ALTIMETER. I FOUND L PITOT AIR DATA MODULE TO BE FAILED (HARD FAULT). REMOVED AND REPLACED SAME. I PERFORMED TESTS PER AMM AND FIM; REF LOG PAGE XXX. INSPECTION VERIFIED QUICK DISCONNECT FITTING; REF LOG PAGE XXX PER GENERAL MANUAL XXX GENERAL PROCS. BY NOT READING PAST THE GENERAL REQUIREMENTS FOR PITOT/STATIC LEAK CHKS IN GENERAL MANUAL XXX; I FAILED TO DO A SYS LEAK CHK FOR AIR RETURNS PER GENERAL MANUAL STEP 4. ACTION TAKEN TO CORRECT THIS: SHIFT MGR/MAINT CTL CONTACTED LINE MAINT TO PERFORM REQUIRED LEAK/PRESSURE CHK; SAME NIGHT. SUPPLEMENTAL INFO FROM ACN 808916: I WAS CALLED TO ACFT XXX BY AVIONICS TO WORK AN RII ITEM AT THE GATE. I MET WITH XXX AND HE NEEDED FOR ME TO LOOK AT THE L PITOT AIR DATA MODULE QUICK DISCONNECT FITTING. I INSPECTED IT FOR BEING PROPERLY SEATED AND LOCKED. I CHKED THE GMM UNDER CHAPTER 34; NAVIGATION/PITOT STATIC SYS. IT SAID FOR QUICK DISCONNECT FITTINGS IS NOT REQUIRED. AT THIS TIME I BELIEVE I HAVE ACCOMPLISHED ALL THE REQUIRED INSTRUCTIONS AND SIGN THE LOG PAGE. LATER THAT NIGHT ABOUT XA00; I WAS TOLD BY MY SUPVR AND SHIFT MGR THAT A LEAK CHK WAS REQUIRED FOR AIR RETURNS. SINCE THE GMM RII LIST CHAPTER 34 DOES NOT SAY ANYTHING FOR PIREPS OF THAT AREA BEING AN RII ITEM; I BELIEVED I DID NOT HAVE TO DEAL WITH THAT WRITE-UP. I ALSO POINT OUT IN THE GMM LEAK TEST SAYS-- EXCLUDING DISCONNECT; SO AT THIS POINT YOU WOULD BELIEVE YOU ACCOMPLISHED YOUR INSTRUCTION.