I've always doubted that rear disk brakes would actually stop the vehicle any better than rear drums when cool.
Here is some info that might surprise you since it is usually taken as a given that rear disks are better:
From:
http://en.wikipedia.org/wiki/Drum_brake Drum brakes have a natural "self-applying" characteristic, better known as "self-energizing." The rotation of the drum can drag either one or both of the shoes into the friction surface, causing the brakes to bite harder, which increases the force holding them together. This increases the stopping power without any additional effort being expended by the driver, but it does make it harder for the driver to modulate the brake's sensitivity.
Disc brakes exhibit no self-applying effect because the hydraulic pressure acting on the pads is perpendicular to the direction of rotation of the disc.
Due to the fact that a drum brakes friction contact area is at the circumference of the brake, a drum brake can provide more braking force than an equal diameter disc brake. The increased friction contact area of drum brake shoes on the drum allows drum brake shoes to last longer than disc brake pads used in a brake system of similar dimensions and braking force. Drum brakes retain heat and are more complex than disc brakes but are often the more economical and powerful brake type to use in rear brake applications due to the low heat generation of rear brakes, a drum brakes self applying nature, large friction surface contact area, and long life wear characteristics(%life used/kW of braking power).
Although drum brakes are often the better choice for rear brake applications in all but the highest performance applications, vehicle manufactures are increasingly installing disc brake system at the rear wheels. This is due to the popularity rise of disc brakes after the introduction front ventilated disc brakes. Front ventilated disc brakes performed much better than the front drum brakes they replaced. The difference in front drum and disc brake performance caused car buyers to purchase cars that also had rear disc brakes. Additionally rear disc brakes are often associated with high performance race cars which has increased their popularity in street cars. Rear disc brakes in most applications are not ventilated and offer no performance advantage over drum brakes. Even when rear discs are ventilated, it is likely that the rear brakes will never benefit from the ventilation unless subjected to very high performance racing style driving.
