SA227 FLC EXPERIENCED SLOW PRESSURIZATION LEAK CLBING OUT OF BIL.

Narrative

I WAS THE PF AND THE FO ON AN SA227 FOR A COMMUTER AIRLINE. WE WERE ENRTE TO WOLF POINT; MT; CLRED TO FL210; STILL CLBING. 60 MI NE OF THE BIL VOR; THE CABIN ALT ANNUNCIATOR LIGHT CAME ON CLBING THROUGH FL207. THE CAPT REQUESTED A RETURN TO BIL AND DSCNT TO 10000 FT. ATC APPROVED. I INITIATED A RAPID DSCNT ALTHOUGH NOT OF THE EMER VARIETY. WE STILL HAD CABIN PRESSURE -- IT JUST HAD STARTED CLBING THROUGH 10000 FT WHICH SETS OFF THE ANNUNCIATOR LIGHT. WE HAD A SLOW LEAK. WE COVERED THE ACTION ITEMS FOR A CABIN ALT ANNUNCIATOR LIGHT AND EACH PERFORMED VARIOUS TASKS DURING THE DSCNT. THE CAPT WAS BUSY READING THE CHKLIST AND HANDLING COMS. MY DSCNT RATE AND MOMENTARY INATTN CAUSED ME TO FLY THROUGH 10000 FT ASSIGNED; TO 9300 FT. WE IMMEDIATELY CLBED BACK TO 10000 FT; BUT NOT BEFORE ATC QUERIED US ON OUR ALT. BY THAT TIME WE WERE GOING THROUGH 9500 FT TO 10000 FT AND RPTED SUCH. ZLC SAID 'ROGER AND HANDED US OFF SHORTLY AFTER TO BIL APCH. WE INITIATED THE ILS TO RWY 28R AND LANDED WITHOUT FURTHER INCIDENT.