What was found
LEFT ENGINE BLEED PRESSURE FLUCTUATED BETWEEN 0 AND 40 PSI. LOST CABIN PRESSURIZATION, RECEIVED A CABIN ALTTITUDE WARNING. RAN QRH , DESCENDED TO 1000 FT AND 315 KTS, HAD EVENTUALLY 1.4 CABIN DIFFERENTIAL 5000 FT CABIN ALTITUDE. LT ENGINE BLEED PRESSURE 24 AND RT AT 36 PSI. TROUBLESHOT IAW FIM 36-10, FAULT TREE 805. REMOVED AND REPLACED THE LT ENGINE PRSOV IAW AMM 36-11-19-4. REMOVED AND REPLACED THE LT ENGINE RFCC IAW AMM 36-11-18-4. PERFORMED AN OPS CHECK OF RFCC IAW AMM 36-11-18-4, SYSTEM OPS NORMAL, PERFORMED A HIGH POWER RUN AND NO FLUCTUATION IN THE BLEED PRESSURE, ACFT OK FOR RETURN TO SERVICE.