ACFT EQUIP PROBLEM MANDATES A RETURN LAND.

Narrative

DURING CLB THE PRESSURIZATION SYS ON OUR MLG FAILED AND CAUSED AN OVER PRESSURIZATION TO OCCUR AT ABOUT 11000 MSL. WHEN THE PRESSURE RELIEF VALVES OPENED; IT WAS QUITE A SURPRISE! NEITHER I NOR MY FO EVER EXPECTED A PRESSURIZATION PROBLEM AT 10000 FT! MY STANDARD SCAN INCLUDES A CHK OF THE CABIN RATE OF CLB GAUGE ON TKOF TO BE SURE THE CABIN STARTS TO PRESSURIZE; BUT AFTER LIFT OFF IN TERMINAL AREAS IN VFR CONDITIONS; I INSIST THAT BOTH OF US BE 'LOOKING OUTSIDE' AS MUCH AS POSSIBLE. THIS WAS THE CASE DURING THIS CLB. ADD TO THIS THE TIME SPENT LOCATING TCASII TARGETS AND YOU HAVE NO TIME FOR SYS MONITORING! WE ASKED ATC FOR AN IMMEDIATE LEVEL-OFF AT 13000 THEN A DSCNT TO 10000. I'LL BE HONEST; I THINK I STAYED CLOSE TO 13000 FT BUT THERE MAY WELL HAVE BEEN AN ALTDEV. IT WAS MY LEG AND I TOLD THE FO TO HANDLE THE PRESSURIZATION PROBLEM FULL TIME. I WAS VERY CONCERNED ABOUT HAVING THE PRESSURE REBUILD AND HIT THE PRESSURE RELIEF VALVES AGAIN BECAUSE OF THE RAPID PRESSURE CHANGES AND POSSIBLE INJURY TO EARS. THE WORKLOAD WAS INCREDIBLE! THE MGMNT OF THE MANUAL PRESSURIZATION CTL WAS A 200 PERCENT JOB FOR THE FO AND I WAS FLYING A 'SINGLE PLT' OP! ALSO WHAT COMPLICATED THE PROBLEM WAS THE TIME IT TOOK TO INTERPRET THE PRESSURIZATION INSTS AND DETERMINE JUST WHAT WAS HAPPENING. THE MLG HAS MANY WARNING AND CAUTION SYS THAT TEND TO 'FEED' YOU THE PROBLEM VERY QUICKLY; HOWEVER; AN OVER PRESSURIZATION IS NOT ONE OF THEM! THE FIRST INDICATION YOU HAVE SOMETHING IS VERY WRONG IS WHEN YOUR EARS TELL YOU. THE INST INDICATIONS ARE INITIALLY VERY CONFUSING! WHEN YOU FIRST LOOK; THE CABIN RATE OF CLB IS PEGGED IN A CLB (BECAUSE THE RELIEF VALVES ARE OPEN) BUT YOU THEN LOOK AT THE CABIN AND IT SHOWS A MINUS ALT; BUT WAIT; YOU'RE NOT SURE BECAUSE IT'S OFF SCALE. THEN THE CABIN DIFFERENTIAL PRESSURE IS HIGH BUT DOESN'T LOOK AT FIRST TO BE EXCESSIVE. THEN WHAT HAPPENS? AS THE RELIEF VALVES START TO CLOSE; THE CABIN AGAIN STARTS TO DSND AGAIN! IT TOOK THE FO FULLY 5-6 MINS TO GET CTL OF THE CABIN. WE RETURNED TO SFO AND LANDED UNPRESSURIZED. THE PAX HAD NO INDICATIONS OF INJURIES OR ILL EFFECTS. I HAVE NEVER THOUGHT MUCH ABOUT AN OVER PRESSURE CONDITION; HOWEVER; ON A 2 MAN COCKPIT; IT SHOULD BE MANDATORY TO HAVE A WARNING SYS TO ALERT YOU TO WHAT THE PROBLEM IS.