If I fill the transmission to the mark on the dipstick the grease will be to the top of the brake shafts. This thing is 35 years old and the chances of holding grease with these seals is slim to none. I have changed seals with little success. They still seep into the brakes. Will there be any damage done if I maintain the grease level at the bottom of the seal? This will be 2" below the dipstick mark.

not sure I would try running it that low, as to the seals my 580CK is at least 5 years older than yours and I have zero seal leaks, when you have replaced seals before did you check the endplay on the carrier assembly? that or damaging the seals on installation should be the only reason they would still leak, unless for some reason the shaft has been damaged/groved ect, I have rebuilt many 580 series brakes/seals and have yet one to leak when I am done, but I have been doing them since 1966 so I do have lots of prior dealings with them,,, as I have said before I always change out the 80wt/90wt with Hy Tran oil and am very happy with how well it works cnt