ATA 26-16, Lower Cargo Smoke Detection

Quite a few 767's that have been converted to freighters have a modified fwd and aft cargo smoke detector system.
  
     
  
Each cargo hold has two detectors. Air is drawn into the detectors from lines (orange in this case) that begin near the top of the compartment in several locations. Two blower motors (although they're really sucking) are located below the detectors and the (negative) pressure switch (gold colored disk in image above).
  
     
  
Only one blower is used at a time. If the switch senses a lack of vacuum (or lack of air flow) for 60 seconds, it will activate the secondary blower motor. A EICAS message will be generated to indicate the failure.

The outlet of the blower motors are located on a common mount plate.
  
     
     
  
On the back side of this plate opening is a thin rubber flapper. Because they are mounted to the same basic "plenum", while one blower is operating, the flapper on the other motor is pulled shut.
  
     
  
As we recently found out..... if the flapper of the idle blower fails to close (falls off, has holes, etc...) air will be drawn through the opening to the operating blower. There will be no air drawn through the detectors and the pressure switch will cause a fall-back to the secondary blower.

The explanation above is wrong dealing with the flapper.

When a blower is operating..... it pulls "its own" flapper flush to the mount. This isolates the plenum and the secondary blower system from the airflow. All air is drawn from the tubes on the cargo pit ceiling. Air is pulled through "both" sensors and then exits the running blower.

In other words..... it the flapper fails on the running blower, air is drawn backwards through the non-running blower and not through the tubes and sensors.