Weeping Differential Breathers

My '04 T2000 project truck has a chronic problem with fluid leaking out of the differential breathers (the ones that look like bottle caps). It's occurring on both drive axles, the rear being the worst. Looking at the past service records it has been noted as far back as 2 years ago, and is a chronic problem.

The truck gets a B service (comprehensive PM) every 15k. Last time they put 11 quarts into the rear most differential. I think the capacity is only what 18? 20? So in other words, it's leaking a lot from the breathers.

If it had a cracked case and water was getting in, the shop would notice the milkshake during service, yes? They aren't overfilling the pumpkin either- I checked.

My W900L doesn't do this, nor did my Columbia. Any idea wtf is causing this to happen?

Thank you.

 

Thanks, I'll start there. Appreciate ya.

 

When I checked it, I used the upper plug on the diff. housing. Not sure how the shop has been doing it, but it looks like the same way due to the clean areas.

 

Good to know! Kenworth probably was doing it right. I just barely opened the plug to make sure it wasn't overfilled- I'm not filling there.

It does sound like the rubber seals are toast in the breathers, so I'll try those new style dana-spicer ones TM suggested.