A B737-200 ON A MAINT FERRY DECLARED AN EMER DUE LOSS OF 'A' SYS HYD PRESSURE AND QUANTITY. CAUSED BY LOOSE 'B' NUT ON INTERIM REPAIR.

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ARRIVED ZZZ1 ABOUT XA30. ACFT HYD FLUID WAS LEAKING FROM THE L WHEEL WELL AREA. FOUND 'A' SYS PRESSURE RETURN LINE WORN THROUGH FROM A BROKEN CLAMP. NO NEW LINE WAS AVAILABLE. WE HAD THE DOWN LINE HOST KIT WITH US. REMOVED THE DAMAGED LINE AND THEN PROCEEDED TO CONNECT 2 LINES TOGETHER TO GET THE PROPER LENGTH FOR A TEMPORARY LINE REPLACEMENT. WE BORROWED 3 MS-8 UNION FITTINGS. HOOKED THE LINES TOGETHER WITH MS FITTINGS AT EACH END. INSTALLED LINE AND TIGHTENED ALL 'B' NUTS TO THE ORIGINAL ENDS OF THE HARD LINES. SVCED THE HYD SYS WITH HYD FLUID. I WENT UPSTAIRS AND RAN THE #1 ENG FOR 2-3 MINS; WORKING THE AILERON AND ELEVATOR CTL SURFACES. LEAK CHKED ALL THE FITTINGS DURING ENG RUN AND NO LEAKS WERE FOUND. AGAIN; SVCED THE HYD RESERVOIR FOR LOW QUANTITY. THE ACFT DEPARTED TO ZZZ. ABOUT 20 MINS AFTER TKOF; THE PLTS NOTICED THAT THE HYD QUANTITY WAS LOW. BY THE TIME WE WERE IN THE AREA; THE LOW PRESSURE LIGHTS CAME ON. THE 'A' SYS HAD FAILED DUE TO NO HYD FLUID. LNDG WAS NORMAL AND THE ACFT WAS TOWED OFF THE RWY. UPON MY INSPECTION; I FOUND THE UPPER UNION 'B' NUT LEAKING. I TIGHTENED THE 'B' NUT ABOUT 1 FLAT MORE AND THE LEAK STOPPED.