Warning: Actual C4 tech question.
 

So I dropped my exhaust to install headers (yay). While it is off, I want to investigate my ZF tailshaft leak. Ok, so I'm thinking the rear seal is bad, but I sprayed it down with brake cleaner twice and it still leaks. I haven't started the engine in like a month now (I know, I'm lagging on this project).

The seal is dry as can be, but the oil keeps coming from the yoke center- like along its axis. Is that even possible? Is there a hairline crack or something?

http://i29.photobucket.com/albums/c2...ps8c6fb3ea.jpg

http://i29.photobucket.com/albums/c2...ps92e8357a.jpg


maybe its running from the top, but mmm, for a month now? I clean it up and 3 days later the drops hit the garage floor/

So looking at the pic, the oil seems to come from the flat surface 3/8 inch from the u-joint. I want to call Bill B (the ZF Doc) to confirm, but this would be the first time in my life I ever saw bad d/s yoke metallurgy.

Don't feel bad, I still wonder where half of one quart went with no leaks at all.

Drive the Z until it reaches Op Temp and look at it again.

It has been leaking for a year. The pipes burn it off. I hope it needs a new yoke.

I assume the seal is new....back when you fixed the 2nd gear.

May be some porosity in the metal. Seen it before....unfortunately, the only way I know of to really test this is to replace the yoke and see what happens.

It is and the yoke bushing in the tailshaft is new also. I was pretty sure the seal was bad or there was too much play- but surprisingly, this is what I found.

Can a machine shop polish it?

Possibly.

My first real experience with metal porosity was on the wheel on my 84. Was losing about 5 psi per week. Turned out the metal was porous. I had the tire dismounted and applied a thin coating of JB Weld over the spot. Problem was solved. Probably not possible with a drive shaft yoke though.

I would think a machine shop could check it for cracks too. It's pretty simple to do.

Metal porosity can be a real bitch. Once when I was racing I had a block bored .030 over. Everything checked out good so I put it together, started it, set the timing and let it warm completely up. I then shut it down to pull the valve covers & reset the valves. Got that done put the covers back on & went to start it again, it was locked up! I ended up tearing the heads off again to see what went wrong and I could not find a thing. Cleaned out all the cylinders so they were dry and went to check gaskets, came back & #3 cylinder had water in it! I wiped it out again then stood with a light right over the bore. All of a sudden a drop of water started forming on the cylinder wall. That engine was a running engine but when they bored it the hit a spot where there was just no material. The hole could not be seen either.

UPDATE with Pics:

took it apart and could clearly see the freeze plug-like spline end cap is leaking. Not too common, but a quick search on the internet does bring up the phenomenon.
http://i29.photobucket.com/albums/c2...ps537b7588.jpg

I could have probably got away with JB-Weld-
http://i29.photobucket.com/albums/c2...ps444f55f3.jpg

Additionally...

From Bill (ZFDoc) Boudreau;

I did find one genuine replacement yoke in Florida- $195. Decided to fix my 37k mile one.

Damn Mike, that a lot of ca$h, you could have use JB weld, regular welding or anything else. Test it for a day or two, then make a decision.

One thing is you found out what it is and will get it fix soon.

Your simple weld should work fine. No way I'd fork over that kind of cash if it's a simple fix.

Looks like a solid fix. I'm willing to bet your leak will be fixes for ever.