I know it needs a viscous coupler because it is full of gear oil. But I want to know why my rear diff is pressuring up causing the viscous coupler to fill up with oil. That is why I was asking about the vent. Until That problem is solved I don’t want to put a viscous coupler in. Because last spring when the problem started I put a new seal in and brake cleaned the inside of the viscous coupler (I know you are not supposed to do that) but I tried it since I knew i needed a new one any way and it worked until this summer when I noticed oil leaking again.