A 737-300 FLIGHT CREW EXPERIENCES CABIN PRESSURIZATION LOSS. COMPANY MAINT CONTROL HAD CONTRACT MAINT DEFER AND CLOSE #1 ENGINE BLEED AIR S/O VALVE FOR LOW AIR PRESSURE WHEN; IN FACT; #2 ENG BLEED WAS FAILING.

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I MET THE ACFT TO BEGIN DAY 2 OF A PM TRIP. CONTRACT MAINT WAS THERE WORKING ON THE AIRPLANE. THE DEPARTING CAPT HAD WRITTEN UP THE PRSOV. MAINT CTL HAD FAXED DIAGRAMS AND TALKED TO THE PHONE TO ZZZ CONTRACT MAINT AND INSTRUCTED HIM TO CLOSE THE #1 ENG BLEED AIR SHUTOFF VALVE AND DEFER IT UNDER MEL 36-5. THE ACFT ALSO HAD MEL 74-1 #1 ENG L IGNITION SYS. WE LOADED UP AND DEPARTED 7 MINS LATE. MEL 36-5 HAS SOME ALT RESTRS SO WE WERE FILED AT FL230. I READ THE MEL TEXT AND CONFIGURED FOR TKOF WITH THE L PACK USING APU AIR; THE R PACK USING THE R ENG BLEED AND THE ISOLATION VALVE CLOSED. WE CLBED UNEVENTFULLY TO FL230. ONCE THERE; I REALIZED WE SHOULDN'T BE USING THE APU FOR BLEED AIR ABOVE 17000 FT SO WE RECONFIGURED (PER MEL TEXT) TO L PACK USING R ENG BLEED; R PACK OFF; AND THE ISOLATION VALVE OPEN. SHORTLY THEREAFTER; THE ALT WARNING HORN SOUNDED. WE ASKED ATC FOR LOWER; BUT COULD NOT DSND IMMEDIATELY BECAUSE OF XING TFC 1000 FT BELOW. THE CABIN WAS SLOWLY CLBING; SO WE IMMEDIATELY PUT THE APU BLEED BACK ON AND WHEN ALLOWED BY ATC STARTED A TURN AND DSCNT. IT TOOK SEVERAL MINS FOR THE ALT WARNING HORN TO SILENCE. AFTER LEVELING AT 17000 FT; WE CONFIGURED WITH L PACK ON APU BLEED; R PACK USING R ENG BLEED AND ISOLATION VALVE CLOSED. SUPPLEMENTAL INFO FROM ACN 762722: THE CABIN PRESSURE DID NOT EXCEED 10000 FT AND THE PAX OXYGEN MASKS DID NOT DROP. CONTINUE WITH ACN 762328: WE CONTINUED ON TO ZZZ1 UNEVENTFULLY. COMPANY MAINT MET THE ACFT IN ZZZ1 AND IN GOING OVER THE ORIGINAL LOGBOOK WRITE-UP REALIZED THAT THE R ENG BLEED VALVE WAS THE ONE THAT HAD LOW PRESSURE AND SHOULD HAVE BEEN CLOSED. THE LOGBOOK ENTRY WAS 'SIGNIFICANT PNEUMATIC DUCT PRESSURE SPLIT WITH ISOLATION VALVE IN AUTO; AT CLB PWR; R SIDE PRESSURE 8 PSI AND L SIDE 46 PSI.' WHEN I WAS TALKING TO CONTRACT MAINT AT OUR DEP CITY; I ASKED IF HE WAS SURE HE WAS WORKING ON THE CORRECT ENG. HE LAUGHINGLY SAID; 'WOULDN'T THAT BE FUNNY' AND ASSURED ME THAT HE WAS DOING EXACTLY AS INSTRUCTED BY MAINT CTL. CONTINUE WITH SUPPLEMENTAL INFO FROM ACN 762722: RPTEDLY MAINT CTL HAD THE INFO REVERSED FROM WHAT WAS IN THE LOGBOOK. AFTER FURTHER DELIBERATION AND CONSULTATION; IT WAS DETERMINED BY ZZZ1 MAINT PERSONNEL THAT WE SHOULD RUN THE ENGS TO BE CERTAIN WHICH ENG HAD THE LOW PRESSURE. AFTER THE ENGS WERE RUN; IT WAS VERIFIED THAT THE R ENG HAD THE LOW PRESSURE. THE ZZZ1 MAINT PERSONNEL ENSURED BOTH ENG BLEED VALVES WERE CORRECTLY SET. AFTER THE APPROPRIATE LOGBOOK ENTRIES WERE MADE AND THE MEL CHANGED TO 36-5 FOR THE #2 ENG; WE COMPLETED OUR PREFLT DUTIES AND PUSHED BACK 57 MINS LATE FOR THE FLT TO OUR NEXT CITY. THE ACCEPTING CREW WAS BRIEFED. ACFT NUMBER WAS SCHEDULED TO TERMINATE IN ZZZ1 AFTER THAT 1 LEG BACK. THE WORDING IN THE MEL CAN BE CONFUSING. SUGGEST THAT THE WORDING BE REVIEWED FOR CLRER PHRASING.