Transmission speedo thing leaks... replace or repair?

[Skunksmash]

That weird thing that sticks out of the side of my 700R4 in my 87 is leaking. Its not just a speedo sensor, it is said that it also can somehow adjust for different tire sizes. I've found them here and there, they sell them at autozone and orielly's and such.

But I've never replaced one so I thought I'd ask. Do those things usually leak? Or does the leaking indicate some kind of internal problem? Is it just some kind of O-ring in there that usually goes bad? Didn't want to buy a replacement and install it before I had some kind of idea of what's going on.