A B737-300 ON DSCNT DECLARED AN EMER DUE TO LOSS OF 'A' SYS HYD PRESSURE AND QUANTITY CAUSED BY A LEAK ON OR NEAR THE RUDDER PWR UNIT.

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ABOUT 80 MI E OF ORD ON THE PMM ARR; WE LOST OUR SYS 'A' HYD QUANTITY. WE RAN THE CHKLIST FOR THE PROB AND DECLARED AN EMER WITH ATC. WE WERE CLRED DIRECT TO ORD. THE CHKLIST FOR OUR PROB WAS TO INCLUDE A MANUAL GEAR EXTENSION. I CALLED THE FLT ATTENDANT AND TOLD HIM WHAT THE PROB WAS AND THEN DIRECTED HIM TO DO A CABIN ADVISORY WITH THE OTHER FLT ATTENDANTS AND THAT WE SHOULD BE ON THE GND IN ABOUT 15 MINS. THE MANUAL GEAR EXTENSION WENT AS IT SHOULD. I TOLD THE FO TO MAKE AN ANNOUNCEMENT TO THE PAX THAT THERE WOULD BE EMER VEHICLES ALONG THE RWY FOR OUR LNDG AND THAT IT WOULD BE CONSIDERED NORMAL UNDER THE CIRCUMSTANCES. THE LNDG WAS NORMAL AND WE TAXIED TO THE GATE UNDER OUR OWN PWR. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED ON PARKING ON THE GATE A TECHNICIAN ADVISED THE COCKPIT NOT TO START THE APU AS THE AREA WAS COVERED WITH HYD FLUID. THE RPTR SAID THE CAUSE OF THE LOSS OF FLUID WAS A LEAK IN OR ON THE RUDDER PWR UNIT. THE RPTR STATED IN CHKING LATER IN THE ACFT HISTORY IT WAS FOUND THE RUDDER PWR UNIT WAS REPLACED.