What was found
WHILE CLIMBING THROUGH 28000, RECEIVED A LT BLEED DUCT WARNING. IT PERSISTED FOR MORE THAN 30 SEC, WHICH LED TO AN UNPRESSURIZED LANDING. FOLLOWED NON-NORMAL CHECKLIST FOR LT BLEED DUCT 8-252 & UNPRESSURIZED LANDING 8-256. DEFERRED LT AIR LEAK DETECTION SYS IAW MEL 36-21-6E, ENTERED INTO MM02 & PLACARD INSTALLED. TROUBLE SHOT, LOOP B LT READS HIGH. REPLACED CONNECTOR MT138P1 & INSULATOR IAW 36-21-01. OMH CHECKED GOOD 49 OMHS. STARTED APU WITH BLEEDS ON. LT LOOP B READS ERRATIC FROM 60 OMHS - 400 OMHS. SUSPECT MT142P1 AT APU FIRE WALL DUCT, ITEM REMAINS DEFERRED. REPOSITION LOOP A AT MT202P1. REPLACED CERAMIC INSULATORS AT MT142P1 & MT203P1 IAW AMM 36-21-00. RAN APU PACKS ON, NO LEAKS, LT B LOOP 52 OMHS, LT A LOOP 43 OHMS. ALL CHECKED GOOD. REMOVED FROM MM02.