What was found
ON TAXI -OUT NOTICED SPLIT ON N1 FOR TAKE-OFF POWER. LOOKED AT DUCT PRESSURE GAUGE AND LEFT DUCT INDICATED ZERO. CONFIGURED APU TO BLEED DUCT AND PRESSURE RETURNED TO NORMAL. WENT BACK TO #1 BLEED ON AND LEFT PRESSURE DROPPED TO ZERO AGAIN. GROUNDED - YES. NO C/B'S OPEN FOR BLEED VALVES- RAN #1 ENG DUPLICATED LH DUCT PRESS ZERO-GVI #1 BAR & CYCLED LH BLEED SW HEARD & FELT BAR SOLENOID OPERATE- WRENCH CKD PRSOV, HSV, ASSOCIATED LINES WITH NO LEAKS OR DEFECTS NOTED. REF B737-800 FIM 36-10 TASK 805 AND AMM 36-00-00-860-807. R/R LH PRSOV IAW B737-800 AMM 36-11-04. GROUND VERIFICATION OK. TROUBLESHOOT PER FIM TASK FIM TASK 36-10 TASK 805 NO FAULTS OR DEFECTS FOUND. AS A PRECAUTIONARY MEASURE REMOVED AND REPLACED THE LEFT ENGINE BLEED AIR REGULATOR REF. AMM 36-11-03. OPS AND LEAK CHECKS GOOD. ACCOMPLISHED BLEED AIR CROSSOVER OPERATIONAL TEST REF AMM 36-11-00.OPS TEST GOOD. GROUND VERIFICATION OK.