FLC OF A DC9-40 DISCOVERED LOW PAX OXYGEN PRESSURE DURING CLB THROUGH FL180 CAUSING CONCERN FOR A SUFFICIENT SUPPLY FOR AN EMER. THEY REQUESTED A LOWER ALT AFTER CHKING FOR ADEQUATE FUEL TO DEST AT THE LOWER ALT.

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I WAS PF AND PASSING THROUGH FL180 PART OF THE CREW 'FLOW' IS TO CHK OXYGEN. SINCE I WAS SEATED CLOSEST TO THE OXYGEN BOTTLES I HANDED CTL TO THE CAPT AND CHKED THE OXYGEN. THE CREW BOTTLE SHOWED FULL; HOWEVER; THE PAX OXYGEN WAS ONLY SHOWING ABOUT 200 PSI. WE COMPLIED WITH THE PROCS OUTLINED IN OUR QRH AND FLOW AND COORDINATED WITH ATC AND COMPANY AND DSNDED TO 10000 FT MSL; RECALCULATED OUR FUEL BURN AND CONTINUED TO MYR WITHOUT ANY FURTHER PROBS. I AM NOT SURE WHEN THE LEAK OCCURRED. HOWEVER; I DID CHK BOTH OXYGEN BOTTLES WHEN I BOARDED THE ACFT. I REMEMBER THIS BECAUSE THE PREVIOUS CREW HAD TURNED THE VALVE ALL THE WAY OPEN TO THE END OF THE THREADS. I TURNED THE VALVE CLOSED JUST ENOUGH TO GET IT OFF OF ITS STOP. AT THIS TIME I NOTICED NO LEAK OR SOUND OF OXYGEN ESCAPING. THE MAINT IN MYR LOOKED AT THE SYS AND AFTER TESTING AND TIGHTENING EVERYTHING CONCLUDED THAT WHEN IT WAS SVCED EARLIER A SMALL LEAK EXISTED FROM NOT BEING TIGHT WHICH CAUSED A SLOW LEAK WHICH WOULD HAVE GONE UNNOTICED IF NOT FOR THE 18000 FT PRESSURE CHK.