Rim & Tyre....can't explain flat.
 

I have '90 Notch and for a while now the front passenger side tyre will progressively deflate over time. For instance it will be completely flat, I fill it up to recommended pressure and about two weeks later it will be flat again. Now I don't drive the car everyday but this is the only wheel that does this. I have original 10 hole wheels. I took it to a tyre shop who inflated it to 50 psi and dunked in a tank to see where it might be leaking but they were not able to find any spots where it was losing air...figured if it's going to show a leak, it'll do it at 50 psi.

A coworker mentioned that it's a common thing with these old wheels. He's saying that they develop a layer of corrosion or crud or something along those lines right along where the bead of the tyre seats against the wheel. Is this true or is he talking out of his ass? If he is right and this is a common ailment, would I be able to simply take some sandpaper to the bead line and clean up the crud?

If not, what else could be case?

Yes, aluminum rims do corrode some at the bead. You'd have to break the bead and remove the corrosion. A slow leak is sometimes hard to see. Putting 50lbs in a tire also might have sealed the leak temporarily. I use soapy water on the bead when I check for leaks, bubbles don't lie. Good luck

Right on, I'll have to give that shot.

Do you think if I let it get completely flat I can push the tire far back enough and get in there to clean without having to remove the tyre completely?

Nope, just take the wheel off and take it to a tire shop, they'll take the tire off, clean rim, reinstall tire for $10-20 maybe. Breaking the bead at home isn't easy even with the proper tools