if you have an air leak with the key on it is one of the seals failed that run the piston to separate the clutch allowing the hub to turn freely when you turn the key off or engage the fan on with a manual fan switch the air leak stops because the hub automatically locks together because they are spring applied (on) and air released (off or free wheeling)
Wow. Very nice video. Is this the type of clutch that's used in Detroit 60 DDEC4 12.7 Litres ? And also, if my hub is currently leaking air with key on, would the rebuild solve this problem ?