SF340 CAPT REPORTS MAIN ACCUMULATOR PRESSURE CYCLING BETWEEN 2000 AND 3000 PSI IN FLIGHT. FLT CREW ELECTS TO RETURN TO LAND; THE SYSTEM HAD BEEN PREVIOUSLY WRITTEN UP.

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WHILE REVIEWING THE MAINT LOG DURING MY PREFLT INSPECTION I NOTICED THAT THE PREVIOUS DAY A CREW WROTE UP THE MAIN HYD ACCUMULATOR FOR PRESSURE FLUCTUATIONS. MAINT REPLACED THE FLAP VALVE AND SIGNED IT OFF. WE LEVELED OFF AT 16000 FT. I NOTICED THAT THE MAIN HYD ACCUMULATOR STARTED CYCLING ON AND OFF EVERY TIME THE PRESSURE WOULD DROP DOWN TO 2000 PSI. IT WOULD GO UP TO 3000 PSI; BLEED DOWN TO 2000 PSI AND THEN CYCLE AGAIN. I BEGAN TO TIME THE CYCLE GIVEN THE HISTORY ON THE ACFT AND THAT IT WAS HAPPENING OFTEN. THE MAIN ACCUMULATOR WAS HAVING PRESSURE FLUCTUATIONS AND CYCLING EVERY 35 SECONDS. THE EMER; OUTBOARD; AND INBOARD ACCUMULATORS WERE STEADY AT 3000 PSI. THE HYD QUANTITY WAS FLUCTUATING BTWN 80-120 CUBIC INCHES. WE CHKED OUR CIRCUIT BREAKERS; NONE WERE TRIPPED. WE THEN CONSULTED THE NON NORMAL HYD CHKLIST AND THERE WAS NO PROC FOR THE PROB WE WERE HAVING. I CONTACTED DISPATCH AND MAINT AND EXPLAINED THE PROB. WE ALL AGREED THAT WE SHOULD RETURN TO ZZZ. DISPATCH GAVE US OUR AMENDED RELEASE INFO AND I THEN CONTACTED ATC AND TOLD THEM WE NEEDED TO RETURN TO ZZZ. I EXPLAINED TO THEM THAT WE WERE HAVING A HYD PROB. THEY ASKED ME IF I WAS DECLARING AN EMER AND I SAID NOT AT THIS TIME. THE REASON I WAITED A FEW MINS WAS BECAUSE I WANTED TO BRIEF MY FO; FLT ATTENDANT; AND THE PAX AND ALERT THEM OF THE SITUATION. I DID NOT FEEL THAT WE WERE IN EMINENT DANGER AS WE HAD HYD FLUID AND THE EMER ACCUMULATOR HAD 3000 PSI. THE FO AND I AGREED THAT THE SAFEST COURSE OF ACTION WAS TO DECLARE AN EMER AND RETURN TO ZZZ GIVEN THE FACT THAT WE WERE HAVING PROBS WITH THE HYD SYS. I CONFIRMED WITH THE FLT ATTENDANT THE PAX COUNT AND REVIEWED OUR FUEL ON BOARD. BY THIS TIME WE WERE HDG BACK TO ZZZ AND THE FO WAS THE PF. I CALLED ATC AND DECLARED AN EMER. I TOLD THEM WE HAD 31 SOULS ON BOARD; 2 HRS OF FUEL AND NO HAZMAT. WE WERE ASSIGNED RWY XR. THE WX WAS VFR AND WE DECIDED THAT AT THE COMPLETION OF THE DSCNT CHK WE WOULD SWITCH CTLS AND I WOULD LAND THE ACFT. I CALLED OPS AND ADVISED THEN TO BE READY TO POSSIBLY TOW THE ACFT IF WE HAD ANY PROBS ON LNDG. I DID NOT HAVE TIME TO CALL DISPATCH BACK AND ADVISE THEM WE HAD DECLARED AN EMER. I DECIDED I WOULD TALK TO THEM AND MAINT WHEN WE WERE SAFELY ON THE GND. I TOLD APCH TO HAVE THE TRUCKS STANDING BY IN CASE WE HAD ANY PROBS ON LNDG. THE FO AND I THOROUGHLY BRIEFED THE APCH AND DISCUSSED ANY POTENTIAL PROBS AND HOW WE WOULD HANDLE THEM. IT WAS A NORMAL APCH AND LNDG WITH NO PROBS. ONCE ON THE GND THE HYD PUMP STOPPED CYCLING. WE HAD ALL NORMAL INDICATIONS AND TAXIED TO THE GATE.