As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ONT- ON FLIGHT 2512, DURING TAXI OUT, THE LEFT BLEED CAUTION MESSAGE ILLUMINATED. AIRCRAFT RETURNED TO THE GATE. MAINTENANCE PLACED THE NR 1 PRSOV ON MEL. MEL CLEARED ON 7/23 BY REPLACING THE LEFT PRSOV. OPERATIONAL CHECKS GOOD. AIRCRAFT RETURNED TO SERVICE.