ACR MDT IN A DIVERSION TO ALTERNATE AFTER EMER DECLARED IN A NIGHT OP.

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WE WERE PREPARING TO DEPART FROM BWI FOR HPN. WE WERE STILL AT THE GATE. MY CAPT AND I HAD FINISHED THE AFTER START CHKLIST AND HAD CALLED FOR TAXI WHEN THE CAPT (PF) NOTICED WE WERE LOSING HYDRAULIC FLUID IN SYS #2. WE RETURNED TO THE GATE. MAINT AT BWI WORKED ON THE ACFT FOR OVER 4 1/2 HRS (NOTE: THERE WAS ONLY ONE MECHANIC WORKING IN THE RAIN; IN POOR LIGHTING). WHEN MAINT WAS FINISHED WE RAN THE ACFT UP; CHKED FOR LEAKS AND TESTED ALL AFFECTED SYS OF #2 HYDRAULIC SYS. ALL TESTS WERE COMPLETED TO THE SATISFACTION OF THE CREW AND MAINT. WE LEFT THE GATE FOR HPN AND APPROX XA10 AM. APPROX 35 MI S OF PHL THE CAP NOTICED A SLOW LOSS OF HYDRAULIC FLUID FROM SYS #1. THIS WAS NOT THE SYS WHICH HAD BEEN SERVICED IN BWI (ALTHOUGH HYDRAULIC FLUID WAS ADDED TO THE SYS TO BRING THE FLUID LEVEL ABOVE MINIMUM DISPATCH LEVEL (1.5 QUARTS) WE DECIDED TO DSND FROM 19000 FT TO 11000 FT AND ENTER A HOLDING PATTERN TO PERFORM THE HYDRAULIC PRESSURE/LOSS QUANTITY EMER CHKLIST. WE ALSO DISCUSSED WHICH ARPT WOULD BE MOST SUITED FOR OUR SIT AT HAND AND AGREED PHL WAS THE BEST CHOICE FOR OUR DIVERSION. (WX CONDITIONS; LENGTH OF RWY; WIND DIRECTION AND AVAILABLE EMER ASSISTANCE IF NEEDED). WE COMPLETED A FULL CREW BRIEF ON THE ILS RWY 9R AT PHL. NOTIFIED ATC (DECLARED AN EMER) BRIEFED THE FLT ATTENDANT ON THE SIT AND OUR INTENTIONS TO LAND AT PHL; BRIEFED THE PAX AND NOTIFIED THE COMPANY OF OUR SIT WE COMPLETED ALL OTHER APPLICABLE CHKLIST (DSCNT; INITIAL AND FINAL (FLAPLESS LNDG) APCH TO LAND). WE TOUCHED DOWN WITHOUT INCIDENT AND TAXIED CLR OFF RWY. AFTER WE AGREED THE #2 HYDRAULIC SYS PRESSURE AND QUANTITY WERE NORMAL (LANDING GEAR IS ON #2 SYS) WE TAXIED TO THE GATE AREA AND SHUTDOWN THE ACFT. THE PAX WERE REMOVED WITHOUT ANY KNOWN INJURIES OR DAMAGE TO ACFT. IN CLOSING I WOULD LIKE TO ADD THAT OF NO TIME DID I FEEL THE OUTCOME OF THIS EMER BE ANYTHING BUT A SUCCESSFUL LNDG WITH NO DAMAGE OR INJURIES TO ACFT OR CREW AND PAX. THE CAPT RAN THE COCKPIT IN A COOL; PROFESSIONAL MANNER USING ALL RESOURCES AVAILABLE (CREW RESOURCE MGMNT) AND ALL DECISIONS WERE DISCUSSED AND AGREED ON BY THE CAPT AND MYSELF. I ALSO FEEL FIRE AND RESCUE CREWS; ATC AND ALL OTHER PARTIES INVOLVED IN THE EMER AT PHL PERFORMED EXTREMELY WELL AND WERE PREPARED TO REACT TO WHATEVER THE OUTCOME.