And brake service becomes a problem too often because that that cover is not well vented. All in all, that whole brake system design is not well thought out. And that is being generous. Yanmar didn't think through the details of how brakes are used and what happens to them when they get wet. So back in the day when these were under warranty, we always did see a lot of un-necessary rust in the Yanmar springs, brake cam, and drums. It's a problem for gray market machines used in rice paddys too.
Those of us who have had to deal with rusty Yanmar brakes are careful not to let it happen again.
It may cost a cover, but you will get it off, and nice brakes are almost worth the effort.
To avoid another "too soon brake job", lathe the rust pits out of the inside face of the drum, replace the shoes with new bonded ones, use new springs, and make sure water can't enter via the brake cam shaft. Oh, and while you have it off, make up a proper vent for the brake cover. You might want to check if the vent on the axle housing behind the seat needs improving as well.
rScotty