Differential air leak

It will be most likely be That the diaphragm in the Power divider has a Hole in It..!!

 

Picture of where your air line connects to the front diif? Some have an air cylinder that unscrews and you just screw in a new one.

 

Hard to tell from your picture but I am guessing that is in the Eaton 380 - 402 series. Believe those have two bolts, one on each side of the air line, which allow the air cylinder to be removed. Piston can removed from shift fork. If the hard parts are good you can just replace the o -ring.
Check your model number.

 

By the year and picture looks like you have the typical DSP40, what that air line screws into is a plastic cap that you can unscrew out of the housing, under that is a plunger, both have o-ring seals on them, replace those 2 o-rings and that should take care of the leaks as long as the cover and plunger aren't cracked.

 

I would like to thank the original poster and all the members for shining light on what was needed for this issue. Even though this thread is one-year-old it was able to help me today before the end of 2023.

I wanted to contribute by telling you guys that the same exact issue happened to my 2019 KW T680 with 1,400,000 miles. as per on the label of the door sill my Differential is DSP40 the part number and the cost for the whole kit instead of just the O-rings is as follow:

Spicer Part no. 510053
This kit includes one rubber O-ring one black plastic washer like and one cover with one screw in the middle.

I’ll try to include the picture