What was found
AT FL 340, 2 10 HR IN FLT, BLEED NR 1 OFF ALERT FOLLOWED 1 MIN LATER WITH LEVEL 3 AIR MANF 1 FAIL ALERT. NR 1 AIR SYSTEM SHOWED 25 PSI STILL IN SYSTEM WHEN ALERT DISPLAYED. 5 MIN LATER, LEVEL 3 ALERT EXTINGUISHED AS PRESSURE DROPPED TO LESS THAN 5. FOUND LT WING DIFFERENTIAL PRESSURE SWITCH FAULT BALL TRIPPED. R & R LT WING DIFFERENTIAL PRESSURE SWITCH IAW MM 36-23-03-4. PERFORMED LEAK CHECK. NO LEAKS NOTED. COMPLIED WITH NR 1 PNEUMATIC SYS DECAY CHECK IAW MM 36-00-01-5. DECAY CHECKED OK. ACCOMPLISHED NR 1 MANIFOLD FAILURE DETECTION PRESSURE SWITCH ADJUSTMENT AND TEST WITH NORMAL RESULTS REF MM 36-23-03-5. OK TO CONTINUE.