What was found
AIR TURN BACK. ALT DISAGREE EICAS MESSAGE AT FL 390. CAPT ALT 39000 , FO ALT 39300 , STBY ALT 39600 PROGRESSIVELY GETTING WORST. PERFORMED TESTS OF THE AIR DATA COMPUTER SYS L AND R SUSPECT LEFT SYS AT FAULT , CHANGED LEFT AIR DATA COMPUTER IAW AMM 34-12-01 AND OPS CHECK 34-12-00 PITOT TEST 34-11-00 ALL CHECKS GOOD . CHECKED DRAINS AND QUICK CONNECTS FOR MOISTURE AND MISSING CAPS.