What's a "dual bowl brake master?" Are you simply referring to the brake master cylinder? Replaced mine recently. Tore apart the old one. Within the cast part there are 2 sections that compress brake fluid. Rear section is larger for the front disk brakes (need more power so is larger). Front section is smaller for the rear drum brakes (need less power to operate drum brakes). The reservoir has one fill hole but is sectioned into 2 parts. If a bad brake fluid leak happens in the rear, the front brakes will still work for a while because it's "section" of the reservoir is separate from the other section. And vise versa.