DHC8 FLT CREW ENCOUNTERS PRESSURIZATION PROBLEM AFTER TKOF. REPORTER INDICATES MAINT MAY HAVE LEFT THE PRESSURIZATION MODE SELECTOR IN THE DUMP POSITION FOLLOWING A MAINT RUN-UP.

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ON CLBOUT; WE EXPERIENCED A CABIN PRESSURIZATION WARNING LIGHT AT 12000 FT. WE REQUESTED A DSCNT TO 10000 FT FROM APCH AND INFORMED THEM WE HAD A PRESSURIZATION ISSUE. WE PERFORMED THE APPROPRIATE CHKLISTS AND FOUND THE AUTO/MANUAL/DUMP SWITCH TO BE IN THE DUMP POS. WE RETURNED IT TO AUTO AND CONTINUED WITHOUT FURTHER INCIDENT. PRIOR TO THIS FLT; WE HAD A MAINT ISSUE WHICH REQUIRED MAINT TO PERFORM AN ENG RUN-UP. WHEN MAINT DOES THIS; THEY OFTEN PLACE THE AUTO/MANUAL/DUMP IN DUMP. WHEN THEY FINISHED THEIR RUN-UP; THEY MUST NOT HAVE RETURNED IT TO AUTO. AS A CREW; WE SHOULD HAVE CAUGHT THE AUTO/MANUAL/DUMP WASN'T IN AUTO DURING OUR BEFORE START CHK WITH THE ITEM; 'CABIN ALT CTLS...SET.' OFTEN TIMES; WE ONLY CHK TO MAKE SURE THE ALT IS CORRECT AND THE CORRECT BAROMETRIC PRESSURE IS IN. DURING THE AFTER TKOF AND CLB CHKS; THE PLANE WAS PRESSURIZING; BUT AT A SLOWER RATE THAN NORMAL; BUT GOING TO ASE IT WILL TAKE THE CABIN TO 8000 FT BEFORE INCREASING THE INTERNAL PRESSURE DIFFERENTIAL. AT 10000 FT; IT SHOWED 8000 FT WHICH WOULD APPEAR NORMAL. IT TOOK THE EXTRA 2000 FT TO GET IT OVER 10000 FT. WE REVIEWED WHAT CABIN ALT CTLS SET MEANS IN THE FLT MANUAL AND RECOMMEND THAT OTHERS DO AS WELL. PERHAPS; PRESSURIZATION CONTROLLER WOULD BE A BETTER WAY TO WORD THIS ITEM SO CREW CHK MORE THAN THE ALT. ALSO; MAINT SHOULD RETURN THE ACFT TO THE CONDITION THEY FOUND IT IN; OR CREWS SHOULD BE REQUIRED TO DO AN ACCEPTANCE CHK AFTER ENG RUN-UPS.