I've noticed on my '67 (327 V-8) that oil seems to be leaking out of the top of the filler tube, where the top piece (the part that holds the cap on) is crimped on to the main tube. Since this upper part is merely crimped or spot-welded in place, it is not an air-tight seal, even with the cap on. Yet, why would oil be slowly dribbling down from just above where the PCV Valve goes into the side of the tube?

Thinking the tube itself was defective, I just replaced it with a new one, and it is also doing the exact same thing. A check of the PCV Valve shows it to be sucking air (while engine is running), and you can rattle it around when the engine is off, indicating a good PCV Valve.

With that said, has anyone else experienced the same issue? I'm at a loss on this one...

 

Seems like you might have a lot of blow-by, rings could be going out on you

 

Do you still have a working crankcase vent, not just the PCV? The 67 had a draft vent tube in the back of the block, behind the intake manifold. Is that still functional, and if not, is there a breather in one of the valve covers now instead so the crankcase can breathe?

 

Hopefully I don't have a blow-by problem, since the engine was rebuilt about 4 years ago, and not a lot of mileage has been put on it since then.

I've also never noticed anything in the back of the block, such as a vent tube, nor is there any kind of breather cap (nor a provision for one) as part of the valve covers. (FYI, the valve covers are the basic OEM orange-painted "Chevrolet" covers that originally came with the engine, from what I can tell).

 

Pictures of engine??

 

If your block is still a 67, there would be a hole in the back of the block. Could be someone along the way didn't know what they were doing and plugged it. You can push a rubber grommet in that hole, and put the pcv valve in back to give the engine a cleaner look. Then make sure you have a vented cap, not a solid cap, in the oil fill tube (no matter which way you run the pcv). If you're now running a solid cap, your crankcase isn't venting in and out, and burping oil out the cap wouldn't surprise me.

 

Interesting theory about an opening near the rear of the engine. Would it be somewhere on the intake manifold? The only plugged opening that I can see is a half-inch plug on the top rear of the intake manifold... which I would presume is for the vacuum hose that would otherwise go to the power brake booster... that is, if I actually had power brakes.

Anyway, here is a picture of the engine...

 

On the block, not intake manifold. The small journal 327's (up to early 1968) had the road draft tube. The later large journal 327's didn't have it.
And you have no flowing crankcase ventilation; no vented valve covers, and a solid oil filler cap. Your PCV is sucking up and out, but no air is getting back in.
Put a vented cap on, and see what that gives you.
Chevy 327 Small-Block- Car Craft Magazine

 

Good information, and it does make a lot of sense. However, the V8 filler tubes only accept the solid twist-on type of cap. In fact, the only vented cap I've been able to find are listed only for the 6-cylinder engines, not the V8's, and those go onto a short pipe that is part of the single valve cover.

I am going to take a close look at the upper rear of the engine block today, to see if there is any sign of something having once been there. In the past, I have not noticed anything, at least yet. I'll compare the photo you linked to, with what mine looks like (thank you so much for that!)...

 

another option would be to replace the valve covers with a newer style with provisions for breather and pcv.