I checked it out last night and I added a 3/8" NPT pipe T in under the pressure sensor switch that mounts in the middle port of the three ports on top of the ARB. The two outside ports are for the front and rear selinoids and air lines to hook up.
Off of the T you put the pressure switch back in and then install an air chuck fitting.
This allows you to use the ARB compressor for airing up tires etc. but if you loose the comprssor you can always hook up a CO2 tank to the chuck and this back feeds the system giving you lockers agian.
IMPORTANT NOTE: ARB LOCKERS CAN"T USE OVER 80 PSI. IF YOU PUT MORE PRESSURE TO THEM YOU WILL BLOW OUT THE O-RINGS INSIDE THE DIFF.