If your Old Weaver is one made in El Paso (it will say on the scope) it wasn't filled with anything but dry South Texas Air.
It will fog and I don't think Weaver will fix it. Just put it in a dry place with one of those moisture soaking up packages (don't know what they are called).
Lot of demand for those old scopes in CMP Vintage Military Rifle Circles.
I've been searching for one for a 1903A3/4 Sniper rifle project. Finely found a K2.5 with post sight. Took a while to find the Vertical Split rings.
To give you an idea what I'm talking about. To mount this set up you have to remove the rear eye piece of the scope to get the rings on.
I'm sure if I shoot it in the rain its gonna fog, but that's part of the game.
If you just want a Weaver for hunting, get the newer K4s. They're sealed, reasonably prices and hold up quite well. I have one on my Model 70 - 375H&H and it holds up quite will.
If you want to build an vintage rifle keep and use your older model, it will work. If you don't like the idea of fogging, you can go to the CMP forums and find a buyer.
Personally I like the first option, I love old rifle stuff and older El Paso Weavers fit that category.
Or If I could find an old Pre-64 Model 70 in 30-06, the old (fogging) El Paso Weaver K-4s would be my first choice for scoping it.
Just my opiniond but I'll admit, I'm an odd one.
CPT USAR Ret
USAMU Sniper School Oct '78
Distinguished Rifle Badge 1071