A B737-200 RETURNED TO THE FIELD DUE TO LOSS OF 'B' SYS HYD FLUID.

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ON MAR/XX/97 I AND AVIONICS TECHNICIAN ABC UNDERTOOK THE TASK OF TROUBLESHOOTING AN AUTOPLT PROB ON A B737-200. THIS WAS ACCOMPLISHED AT THE GATE FROM XA00 TO XC30. AFTER SEVERAL CHKS WE DECIDED TO CHANGE THE 'B' SYS AILERON PCU XFER VALVE. ABC HELD THE FLASHLIGHT WHILE I DID THE WORK. THE VALVE WAS INSTALLED PER B737-200 MAINT MANUAL. THE PCU XFER VALVE LEAK CHK WAS GOOD; BUT THE AUTOPLT REMAINED INOP ON 'B' SYS. WE DECIDED TO LEAVE THE XFER VALVE IN THE ACFT DUE TO TIME FACTOR AND FOR PRECAUTIONARY REASONS. THE AUTOPLT REMAINED ON MEL. ON MAR/XX/97 OTHER MECHS IN ZZZ REPLACED THE 'B' SYS PCU SHUTOFF VALVE AND THIS FIXED THE AUTOPLT. WHEN THE ACFT DEPARTED ZZZ ON CLBOUT IT SUFFERED HYD FAILURE DUE TO LOSS OF 'B' SYS FLUID. THE CAUSE WAS FOUND TO BE THAT THE XFER VALVE I HAD INSTALLED STILL HAD A SHIPPING GASKET ON IT. THIS IS NOT A PLASTIC CAP BUT A RUBBER BLACK SEAL (WITH HOLES IN IT) OVER THE SEAL PLATE. I KNEW THIS WAS ON THE VALVE; BUT AT THE TIME I DID NOT KNOW IT WAS TO BE REMOVED -- ESPECIALLY WHEN THE OLD VALVE WAS INSTALLED WITH ONE ALSO. THIS LED ME TO BELIEVE INSTALLATION WITHOUT IT WOULD SACRIFICE THE UNIT'S INTEGRITY. THE MAINT MANUAL AND IPC WERE NOT VERY CLR.