Have you ever re-oiled it?
I have a genuine Dri-z-bone that I bought to go to AgQuip (always rains at AgQuip) in 1986.
In '88 I took it on a 3 day hike (not the best choice of hiking apparel, I will admit
) and after the 2nd day of solid rain it started to get a bit damp on the inside.
It got re-oiled shortly after that and I gave it a good coating as it was a bit light on from the factory and it hasn't leaked since.
I like them, a good long coat that keeps you dry and warm as a bonus. Great for around the yards, horses and walking in the paddocks. For use when getting in and out of vehicles, not so much. Horses don't care if you rub grease off onto the seats, the inside of the car suffers a bit though and tucking your skirt up so you can shut the door is inconvenient.
Mine and my dads old coats keep the dust off the drill press. Since moving to Qld its generally too warm for a flannel lined coat when it only seems to rain in summer.