I've been posting here recently about replacing the axle seals and axles in our C5. I blew an axle seal last week, and by blew, I mean in 30 miles, the back of the car and the drivers side exhaust was covered in gear oil. So with some help from the forum, I've decided not to just replace the seals. I'm also concerned about that diff vent being plugged. I've found a few posts on the forum covering it, A lot of the posts suggest a mod - pulling the top hat, spring and rubber seal out of the original diff vent and running a hose with a separate diff vent. That was my approach, but I ran into a problem.
I was able to pry off the top of the vent with a pair of small needle nose pliers. The problem was that I could feel that the amount of "tube" to attach a hose to felt insufficient. Like almost nothing.
At this point, I already had my diff side covers off. Even tho I'm working on a 4 post lift, I still couldn't get my head up there to see it. I did get a bit of a look at it with a mirror and I got a pic with my camera, but those confirmed that there wasn't enough length to securely clamp a hose to. With the driver's side diff cover off, I could look up into the diff and see the bottom of the vent. That's it just to the right of the axle in the pic.


I had a piece of 3/8" dia steel bar in my roll away, and was able to easily tap on and move the vent from the bottom. I was just trying to move it out a bit for a better length of material to attach the hose to, but it popped out of the diff after a couple of light taps. Here's a pic of the pieces of that vent.


As you can see the body of the vent's top shoulder that a hose would attach to is very short. It measures about .218" or 7/32" in length, and it's about .437 dia. We got our C5 in 2006 with 32K miles on it, and I'm pretty sure that's the vent it left Bowling Green with. I would not feel comfortable clamping a piece of 3/8" hose to that short length of 7/16" "tube".
I had ordered up a new vent that I thought I wouldn't be using as I had decided to do the hose mod. After the old vent came out and I got a good look at it, I decided to just use the new vent. Before installing it, I blew through it to make sure it was open. It was, but it wasn't wide open, it was a bit restricted, but consistent and seemed to be designed that way.
I oiled up the base of the new vent, and used a piece of 1-1/2" x 5" long brass round bar to tap the new vent in. Luckily it tapped right in.
So- the answer to the question - Can I replace that vent in the car? - is yes, but you'll likely need the drivers side diff cover off.
And - Should I do the hose mod? - Based on the dimensions of my original vent - I would say there is an insufficient amount of 7/16" "tube" to secure a piece of the recommended 3/8" hose to.