I have this same bike and had a tough time adjusting the brakes until I read the directions in an insert that was in the original owner's manual. If you don't have the manual, PM me with your email address and I can send you a jpg of the two page insert. As far as keeping the brakes clean, Overland used to make a canvas cover that went over the brake mechanism. Works pretty good. It's basically a triangle-shaped sleeve that slides over the brakes from the top and is held in place by a velcro strap. Would be very easy to sew up if they're no longer available. I can send a photo of that too, if you're interested.

Walt