A JUNIOR A&P MECH INSTALLS A HYD LINE IMPROPERLY TO THE L MAIN BRAKE ON A BE1900 WHICH IS FOUND TO BE DISCONNECTED LATER BY THE OPERATING CREW DURING THEIR FIRST POSTFLT INSPECTION AT ZZZ; US.

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ON JUL/XA/01; WHILE DOING A ROUTINE INSPECTION; A YOUNG MECH FOUND THAT THE #1 BRAKE ON THE L MAIN LNDG GEAR WAS WORN BEYOND LIMITS. AFTER THE MAINT WAS COMPLETE; AS AN INSPECTOR; I DID MY WALKAROUND INSPECTION PRIOR TO RELEASING THE ACFT BACK TO AIRWORTHINESS STATUS. DURING THAT WALKAROUND; I FOUND THE ACFT TO BE IN AN AIRWORTHINESS STATUS; WHILE PAYING CLOSE ATTN TO THE MAINT THAT HAD BEEN PERFORMED THAT EVENING; AS IT WAS A NORMAL ROUTINE INSPECTION. THE #1 BRAKE APPEARED TO BE INSTALLED PROPERLY AND APPEARED SECURE. I PHYSICALLY HAND CHKED THE BRAKE LINE GOING TO THE #1/#2 BRAKE; AND ALL CONNECTIONS APPEARED TO BE TIGHT AND SECURE. THE ACFT WAS THEN RELEASED AT XA00 AND WAS TAKEN TO THE LINE FOR ITS MORNING FLT. THE PLTS MADE THEIR FIRST LEG TO GREEN BAY; AND UPON THEIR POSTFLT WALKAROUND INSPECTION OF THE ACFT; FOUND THE #1/#2 L MAIN GEAR BRAKE LINE TO BE DISCONNECTED. THE PLT AT THAT POINT CALLED BACK TO ZZZ AND INFORMED MAINT CTL OF THE SIT. AT THAT POINT; MAINT CTL REQUESTED THAT OUR STATION MAINT GO OUT; RECONNECT THE BRAKE LINE; AND BLEED THE BRAKE SYS. THE ACFT WAS THEN RETURNED TO SVC AND CONTINUED TO FLY FOR THE REMAINDER OF THE DAY. SUPPLEMENTAL INFO FROM ACN 519375: INSTALLED BRAKE ASSEMBLY AT L-HAND #1 MAIN LNDG GEAR AND FAILED TO PROPERLY TIGHTEN THE B-NUT THAT ATTACHES THE BRAKE LINE TO THE BRAKE UNION. THE B-NUT HAD BEEN ATTACHED TO UNION BUT HAD ONLY BEEN FINGER TIGHT. THEN; DURING THE COURSE OF THE JOB; SINCE THE LINE WAS ATTACHED; I FORGOT TO GO BACK AND TIGHTEN THE CONNECTION. THE PROB WAS DISCOVERED AFTER THE PLANE HAD MADE A SAFE LNDG AT ITS DEST AND THE CREW HAD BEEN MAKING A PREFLT INSPECTION BEFORE IT LEFT ON ITS NEXT FLT.