This machine I'm working on had quite a bit of water in compartments. Drain plugs are drilled with cotter pins, but were blocked by mud.

Neil: I asked that question out LOUD more than once; Amazed someone has not done that yet! Around here, probably have to change it every two weeks, even if parked inside. Scary part is I have seen some stuff parked outside for 15-20 years and the inside of the transmission case looked as if someone worked on it the day before. Moisture is the enemy! JM

Here's a picture of the pivot bearings in the top cover of the rusted up 5T(Thank you Juiceman!)I did the clutches on that I was referring to in my earlier reply, I also found drilled plugs with cotter pins but covered by many inches of sludge. Dessicant would work but kind of a pain to maintain. What I do is make replacement 1/8" thick cork/rubber gaskets for all the access covers to make sure they seal well and don't leave your machine out in the weather and if you have no choice then cover it up. It's been working for me so far.Anyway the pivot bearings were frozen rusted to the shafts and the top cover and I figured they were wasted. After finally getting them out with the help of a torch they were packed with rust and pitted on most of the rollers but I threw them in some evaporust for about a week anyway and got them to spin again. Next I hit them with a wire wheel and cleaned them up pretty good then greased em up and reinstalled. They've been working just fine granted I don't run the machine a whole lot but the way I see it is that those bearings rotate what an 1/8 of a turn or so when you pull the levers so it's not like they're working hard at all like a "regular bearing" whizzing around. 
Just my two cents,
CrawlerAddict 

JM the problem is valley / tule fog causes condensation and rusts stuff from the inside out. If you look at water dripping off a metal fence or the water that seems to drip from Valley Oak leaves on a foggy winter day think of what it’s doing inside of your tractor. I have tried those large mil spec desiccant packs and put them in my tool box and they turn solid after a few damp foggy winter days.

I spent last weekend scraping off the rust on my 4 jaw chuck sitting in my in-laws garage for the past 2 years, and was surprised on how much rust there was in that short amount of time.

Hello crawleraddic,
All the gaskets are available from cat and yes they are cork, for the two on trans cover to gearbox (3b889) use 3/8 neoprene round seal cut to length.

Regards

i wonder would a coating of ensis fluid type k made by shell oil. would work to prevent these part from corrosion damage. and as it hydrocarbon wax it wouldnt harm the oil. or prehaps cavity wax or tectyl506

I give credit to CrawlerAddict for making that D4 a runner again. He had the courage to dive into the bowels of that thing and make a runner of it again. Yes, it was one rusty son of a gun. I was amazed it even rolled.
Neil: the majority of my iron stays under cover/inside for a reason here. At your prodding, I think I will go online and see what is available and experiment.
CR: I might have to hook the heaters back up to the old melon shed! We used to store squash in one building here, at it was a constant 58 degrees. JM

From my experience every D4 I've acquired that has had rust in the compartments have all had original gaskets that were no good and or covers that were over tightened and warped enough to cause gaps. I own a lot of other than Cat old iron of similar era with some that has sat outside it's whole life with no corrosion whatsoever in any of the cases or compartments because they were sealed properly. If moisture is not allowed to enter in the first place then problem solved. I really believe it is as simple as that but again just my 2 cents.
JM, Thanks for the mention on the 5T, it's turned out to be one of my favorites!

CrawlerAddict