FLT OF AN MDT DECLARED AN EMER AND DIVERTED TO LAND AT A NEAR BY ARPT DUE TO MAIN HYD SYS MALFUNCTION.

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AT APPROX AFTER COMPLETING ALL CHKLISTS INCLUDING THE AFTER START CHKLISTS WE CONTACTED GND FOR TAXI. (FLT #X TO WHITE PLAINS; NY (HPN)). IT WAS THEN THAT I (PF - CAPT) NOTICED THAT THE NO #2 HYD QUANTITY HAD DROPPED BELOW DISPATCH MINIMUMS (3.0 QTS) AND WAS CONTINUING TO DECREASE AT A RAPID RATE. WE IMMEDIATELY TAXIED BACK TO GATE. MAINT IN BALTIMORE; MD (BWI) WORKED ON THE ACFT (ACTUALLY IT WAS ONE MECH WORKING OUTSIDE IN THE RAIN IN A POORLY LIT AREA). AFTER APPROX 4 1/2 HRS WE WERE ASKED TO RUNUP THE ACFT TO MAKE SURE THAT THE QUANTITY AND PRESSURE REMAINED NORMAL FOR THE #2 SYS. WE ALSO CHKED THE # 1 SYS AS WELL ALTHOUGH WE HAD NOT EXPERIENCED ANY PROBS WITH THE #1 SYS. THE FO AND I WERE SATISFIED WITH THE WORK DONE AS WAS THE MECH. HE SIGNED OFF THE WRITE-UP WITH THE #2 SYS. WE LEFT THE GATE AT XA10 AM FOR WHITE PLAINS; NY. APPROX 35 MI S OF PHILADELPHIA; PA (PHL) WHILE CLBING THRU FL 190 I (PF-CAPT) NOTICED THAT THE #1 HYD QUANTITY HAD DROPPED BELOW 1.5 QTS (MINIMUM DISPATCH LIMIT) AND CONTINUED TO DROP AT A GOOD RATE (NOT AS FAST AS THE RATE DROP OF THE #2 SYS AT BWI). THE FO (PNF) REQUESTED A DSCNT TO 11000 FT AND I CALLED FOR THE LOSS OF HYD CONTENTS/PRESSURE CHKLIST PAGE 18B. WE ENTERED A HOLDING PATTERN AT PHL TO COMPLETE ALL ITEMS OF THE CHKLIST; DISCUSS THE ILS APCH RWY 9R (WX AT THE TIME WAS 200 FT VISIBILITY 1/2 MI); NOTIFIED ATC WE WERE DECLARING AN EMER AND DIVERTING TO PNL; NOTIFIED THE FLT ATTENDANT THAT WE WOULD BE ON THE GND IN APPROX 10 MIN AND TO PREPARE THE CABIN; NOTIFIED OUR COMPANY DISPATCH OF THE PROB AND INTENTIONS. PRIOR TO STARTING THE APCH WE THOROUGHLY DISCUSSED WHAT COMPONENTS WOULD BE LOST WITH THE #1 HYD SYS INOP - FLAPS; ANTI-SKID; #1 INBOARD SPOILERS; NORMAL BRAKES AND PTU. WE BRIEFED FOR A ZERO FLAPS APCH AND LNDG. AFTER TOUCHDOWN WE CLRED THE RWY AND STOPPED. AFTER SOME DISCUSSION (#2 HYD PSI NORMAL QUANTITY NORMAL) WE DECIDED TO TAXI TO THE GATE AREA. WE ARRIVED AT THE GATE AREA AND SHUT THE ACFT DOWN. THE LNDG GEAR WAS PINNED AND THE ACFT SHOCKED. NO INJURIES; NO DAMAGE TO THE ACFT. REFLECTING BACK ON THIS INCIDENT; I WOULD SAY THE FO AND I MADE GOOD USE OF COCKPIT RESOURCE MGMNT. THERE WAS NO DOUBT AT ANY TIME AS TO WHO WAS FLYING THE ACFT AND WHO WAS READING THE CHKLIST AND DOING THE PROBLEM SOLVING. THE UPPER MOST THING IN MY MIND WAS TO MAINTAIN ACFT CTL AND NOT LET OTHER PROBS DISTRACT ME FROM THAT. THE ONLY THING I WOULD HAVE DONE DIFFERENT WOULD BE TO HAVE THE ACFT TOWED TO THE GATE AREA SINCE WE WERE USING THE EMER BRAKE SYS.