A B737-800 CAPTAIN REPORTS OF INFLIGHT CREW OXYGEN FLOW SUPPLY FAILURE. BELIEVES THE 5-SECOND MINIMUM OXYGEN MASK 'PRESS TO TEST' PROCEDURE IS NOT ADEQUATE AND SHOULD BE EXPANDED TO 10-SECONDS.

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PREFLT OF CREW OXYGEN SYS CONSISTED OF THE 'FIRST FLT OF THE DAY' CHKS. OXYGEN PRESSURE INDICATED 1100 PSI (OK FOR DISPATCH) PREFLT CHKS WERE ALL NOTED TO BE OPERATIONAL. CHK CONSISTED OF MIKE AND AIRFLOW CHK WITH SLIGHT DIP IN OXYGEN PRESSURE INDICATOR OF +/-50 PSI. FO LEFT THE COCKPIT FOR A BATHROOM BREAK AND THE FLT ATTENDANT TOOK HER POS IN THE COCKPIT AS SOP REQUIRED. I ATTEMPTED TO PUT CREW OXYGEN ON AND IT INFLATED SLIGHTLY AND I NOTICED A PRESSURE DROP APPROX 500 PSI; SECOND ATTEMPT DEPLETED THE SYS TO LESS THAN 100 PSI. CREW OXYGEN SYS IS INOP. I ADVISED THE FLT ATTENDANT IN COCKPIT TO CALL BACK TO FLT ATTENDANTS AND BRING 2 POB'S UP TO THE COCKPIT IMMEDIATELY. THE FO CALLED THE COCKPIT TO ENTER WITH THE POB'S. I BRIEFED THE FO AS TO THE SITUATION AND WE TESTED THE POB'S AND HAD THEM IN THE 'READY TO USE POS' SHOULD THE NEED ARISE. MEANWHILE; I WAS ALREADY ENGAGED IN A HIGHER THAN NORMAL DSCNT RATE INTO ZZZZ. THE FO ATTEMPTED TO TROUBLESHOOT WITH NO SUCCESS; WE REFERRED TO THE QRH BUT WE FOUND NO CHKLIST FOR THE PROB WE HAD ENCOUNTERED. THE DSCNT TO A SAFER ALT AND THE POB'S ON HAND WAS OUR SUPPLEMENT CHKLIST. I SENT A MAINT MESSAGE TO MAINT CTL TO EXPEDITE THE CORRECTIVE ACTION. UPON LNDG AND AFTER THE POSTFLT CHKLISTS; I WROTE THE CREW OXYGEN SYS UP IN THE LOGBOOK AS STATED; 'CREW OXYGEN NEEDS TO BE RECHARGED TO PREFLT STATUS.' I MET THE CREW AND BRIEFED THEM AS TO OUR EXPERIENCE AND CONTRACT MAINT WAS CALLED IN TO FIX THE PROB. I LATER FOUND OUT THAT THE MAINT HAD FAILED TO OPEN THE CREW OXYGEN VALVE IN THE E&E COMPARTMENT. THE RESIDUAL PRESSURE IN THE LINES GAVE US A SATISFACTORY PREFLT CHK HOWEVER; WE PROVED THAT OUR CREW OXYGEN CHKLIST NEEDS TO BE ALTERED. THE; 'PUSH TO TEST' PORTION OF OUR TEST NEEDS TO BE INCREASED FROM 5 SECONDS TO 10 SECONDS. THIS ADDITIONAL TIME WOULD ENSURE ANY RESIDUAL PRESSURE IN THE LINES WOULD BE ELIMINATED/REMOVED SHOULD THE VALVE BE INADVERTENTLY CLOSED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: REPORTER STATED THEIR CARRIER HAS SOME NEXT GEN ACFT THAT USE STEEL OXYGEN BOTTLES; NOT JUST COMPOSITE TYPE. BUT; THE COMPOSITE BOTTLES SEEM TO BE THE FOCUS OF CONCERN DUE TO THE TEST PROCEDURE FOR THE 'PRESS TO TEST ' OXYGEN MASK SUPPLY MAY NOT BE ADEQUATE ENOUGH FOR THE FLIGHT CREW TO IDENTIFY AN ACTUAL LOW FLOW IN THE SUPPLY LINE FROM A VALVE HANDLE NOT FULLY OPENED. OTHER CARRIERS; INCLUDING HIS; HAVE A MINIMUM 5-SECOND 'PRESS TO TEST' PROCEDURE IN THEIR FLT OPS MANUAL. BUT; THERE IS A FEELING THE 5-SECOND FLOW TEST NEEDS TO BE EXPANDED TO 10-SECONDS. REPORTER ALSO STATED THEIR B737-800'S DO NOT HAVE A CREW OXYGEN SHUT-OFF VALVE IN THE COCKPIT; ONLY IN THE LOWER E/E COMPARTMENT AT THE BOTTLE ITSELF.