I know its not what you want to hear but why the obsession with discs on the rear? Its the absolute bottom of my list of mods. I can lock my 110 up on a dry road. I tow up to the legal limit and the brakes are spot on, all maintained properly and totally stock.
Theres a few points...
1. Properly adjusted drums?
2. Have you rebuilt or replaced the front calipers? Reason for asking is that people never keep up with the reccomended brake fluid changes. When I rebuilt my calipers I found the lower fluid channels were gummed and blocked with rubber seal goop. No fluid was passing. I wonder how many older calipers are like this and reliant on the upper fluid gallery for flow between the two halves of the caliper... the WSM states not to split them so I guess many dont and dont spot this and clear the gallery.
Even vented discs are not that necessary if you arent towing down the alps all day..