Couple weeks ago I posted about my substandard dealership experience trying to fix oil leaking from the rear driver's side wheel. I have updates to report as well as a question. 

Here's the skinny

Car: 2007 Toyota 4Runner, SR5 V8, AWD, 147,000 miles

Problem: Burning oil smell came from driver's side rear wheel. Caliper looked damp like a well-greased cast iron pan. 

1st Service: Replaced rear driver's side caliper --> Leakage problem worsens 

2nd Service: Replaced rear axle seal --> Oil no longer pooling inside the wheel, but leaking continues. Yellow (fresh?) oil found around the caliper and rotor like drops of morning dew. 

Took the car on the highway after the second repair and heard a rattling noise coming by the driver's side door. Aghast, I file a claim to the Toyota headquarters. A case is opened on my behalf. 

Took the car back to the dealership the following day to hear the third diagnosis: rear differential breather was corroded shut, pushing gear oil to leak out. At first, I didn't want the dealership to attempt any more repairs since I was already waist deep in the mess. With the company headquarters now involved, if things went further south the dealership would be shooting themselves in the foot and I didn't want that to happen. However, they were resolute in correcting the problem themselves, so car is at the dealership once more getting a new breather vent along with a bunch of other works redone. 

In his YouTube videos, Scotty makes finding and repairing leaks seem to be a breeze. I see that my situation is slightly different as it involves a supposed root cause -- a small, $10-20 part that could've been screwed on using some pliers. Just had to look a little further upstream to find the vent hard stuck, rather than jiggling like a steam vent from a pressure cooker. 

I believe what's been leaking was gear oil all along. The turning motion of axle likely sprayed oil all over the caliper, damping it from top to bottom. Like @MMJ had commented in my previous post, brake fluid and gear oil were not hard to differentiate. (I was allowed inside the shop area and saw the whole thing.) They are contained in two separate systems; there is no way that fixing one somehow generates stress and causes the other to give out. 

I found many websites that mention differential breather modifications for off-roading, but it seems like its replacement is not exactly part of routine maintenance like the timing belt or other types of fluids. The vent didn't close off overnight and until now there were no warning signs of the vent going bad. Is it commonplace to check the condition of this vent if gear oil is leaking, or is my case a "once in a blue moon" type of problem? 

Thank you again,

JYS