A differential / axle breather serves the important purpose on a 4WD vehicle of venting the air pressure in drivetrain assemblies such as the differentials, transmission and transfer case.  Under normal operations, these assemblies heat up and the air inside expands, increasing the pressure and thus the need to be vented to the atmosphere is created.  When the drivetrain assembly cools down the air inside contracts and creates a vacuum. 

Many drivetrain assemblies are fitted with a breather from the factory.  These factory breathers are usually a one-way valve mounted directly to the housing or remotely on a short tube.  The shortfall with the factory breathers is that driving through water crossings creates a vacuum caused by the drivetrain assembly being cooled rapidly by the water.  This vacuum will tightly close the valve shut causing air to be instantly drawn into the assembly through the seals.  Many times the seals are still submerged below water, leading to the water being drawn into the drivetrain.  This will contaminate the oil and possibly damage drivetrain components or your Air Locker. 
 
The ARB Differential Breather Kit is also unique in that it uses 8mm tubing (where most other kits use 6mm tubing).  The larger size tubing is essential in 4X4 applications where off camber driving can cause oil to enter and block smaller breather lines.

The ARB Differential Breather Kit is designed to eliminate these shortcomings by providing a high flow, zero back pressure, zero vacuum breather system.  This is achieved with a high flow air filter within a central manifold that will allow up to four drivetrain assemblies to be vented to one central location, ideally somewhere on the vehicle that will be above the water level at all times.  This kit is intended to replace your factory breathers and contains enough fittings and tubing to connect at least two drivetrain assemblies.  
 
 
More tubing and push-in fittings can be purchased separately to connect up to four assemblies to the central manifold (Part Number 170113).