Transfer case leak… What is this plug?

[sogncab]

Synthetic has a habit of finding less than perfect seals

 

[anothernz45]

Yes they do. They can get clogged up with debrisView attachment 4109841

They clog up but even when clear they hold a small amount of pressure in the case, they are a one way valve presumably to keep water out when you drive a hot gearbox into cold water. I pick the top cap off, remove the rubber seal then put the spring and cap back together on this one and the diffs. If you are planning on wading one of these old rustbuckets a long pipe with an elbow would work - like a 70. I think the gearbox breathes thru the shifter - don't remember any other.