B737-300 FLC NEGLECTS TO SET THE ENG BLEED SWITCHES TO THE 'ON' POS RESULTING IN PRESSURIZATION PROBS.

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WE HAD JUST BEEN CLRED FROM OUR CRUISING ALT OF 33000 FT TO 31000 FT WHEN I FELT A JUMP IN THE CABIN RATE OF CLB. I LOOKED UP AND SAW THE CABIN WAS CLBING AT 2000 FPM AND THE CABIN WAS PASSING 8000 FT. I POINTED THIS OUT TO THE FO AND WE BOTH CHKED THE PRESSURIZATION CTLR TO DETERMINE WHAT WAS CAUSING THE PRESSURIZATION PROB. EVERYTHING APPEARED NORMAL. WE SELECTED BOTH THE STANDBY AND MAIN POS ON THE CTLR ATTEMPTING TO CTL THE DEPRESSURIZATION AND NOTED THAT THE MAIN OUTFLOW VALVE WAS FULLY CLOSED. AT THIS TIME I THOUGHT THAT SINCE MAINT HAD JUST FINISHED WORKING ON THE APU THAT IT MUST BE A PRESSURIZATION LEAK SOMEWHERE IN THAT AREA. I CALLED THE FLT ATTENDANTS TO THE FLT DECK TO INFORM THEM OF OUR PROB. IT WAS SHORTLY AFTER THEY REACHED THE FLT DECK THAT THE CABIN REACHED 14000 FT AND THE MASKS DROPPED. I TOLD THE FLT ATTENDANTS TO RETURN TO THE CABIN AND ACCOMPLISH THEIR EMER PROCS. I NOTIFIED ATC THAT WE HAD A PRESSURIZATION PROB AND NEEDED TO DSND IMMEDIATELY AND RETURN TO COLORADO SPRINGS ARPT. ATC ASKED IF WE WERE DECLARING AN EMER AND I REPLIED; 'YES.' HE IMMEDIATELY CLRED US TO FL240 AND WE ACCOMPLISHED OUR EMER DSCNT PROCS. ATC EVENTUALLY CLRED US TO 11000 FT. PASSING FL250 I GLANCED UP AND NOTICED THAT THE ENG BLEED SWITCHES WERE IN THE OFF POS. THIS IS BECAUSE WE HAD ACCOMPLISHED A BLEEDS OFF TKOF DUE TO OUR TKOF WT. I IMMEDIATELY TURNED THE ENG BLEEDS BACK ON AND RETURNED THE PRESSURIZATION PANEL TO AUTO REALIZING THIS WAS THE REASON FOR OUR PRESSURIZATION PROB. THE AFTER TKOF CHKLIST HAD BEEN REQUESTED AND ACCOMPLISHED. BUT THE 'AIR CONDITIONING AND PRESSURIZATION SET' WAS MISSED; RESULTING IN THE LOSS OF PRESSURIZATION WHEN THE APU WAS SHUT DOWN AT FL260. THE APU WAS TURNED OFF LATE BECAUSE THE APU EXHAUST TEMP WAS HIGHER THAN NORMAL AT BOTH THE 10000 FT AND FL180 CHKS AND ALSO BECAUSE MAINT HAD SUGGESTED WE MIGHT LEAVE THE APU RUN A LITTLE LONGER THAN NORMAL SINCE IT HAD BEEN INOP FOR SEVERAL DAYS AND HAD BEEN DIFFICULT TO START AFTER THE JUST FINISHED REPAIR. ADDITIONAL TIME WAS TAKEN TO ALLOW THE APU TO COOL. ONCE THE PRESSURIZATION WAS RETURNED TO NORMAL AND A CHK WITH THE FLT ATTENDANTS REVEALED NO PROBS IN THE CABIN THE EMER WAS TERMINATED WITH ATC. PAX WERE BRIEFED AND THE FLT RETURNED TO COLORADO SPRINGS ARPT WITH NO FURTHER PROBS. SUPPLEMENTAL INFO FROM ACN 335525: THIS WAS A CREW ERROR. IT WAS CAUSED PRIMARILY; I BELIEVE; BY THE FACT THAT DURING THE AFTER TKOF CHKLIST THE PRESSURIZATION GAUGES WERE CHKED; BUT NOT THE PHYSICAL POS OF THE SWITCHES. THIS ERROR WAS ALLOWED TO CONTINUE BECAUSE THE PRESSURIZATION WAS OBVIOUSLY PHYSICALLY WORKING AND THE APU WAS DELIBERATELY LEFT ON FOR AN EXTENDED PERIOD. NORMALLY THE ERROR WOULD BE DISCOVERED EARLIER WITH MUCH LESS SERIOUS CONSEQUENCES BECAUSE THE APU WOULD BE SHUT DOWN SOONER. HOWEVER; CHANGING THE AFTER TKOF CHKLIST TO SPECIFICALLY INCLUDE 'BLEEDS ON' MAY HELP INSURE THIS MISTAKE IS NOT REPEATED.