Again, without trying to cause a long debate but still respond to a genuine question.
Maybe I should have been clearer, It is nothing to do with the efficiency of drums over discs on all cars, just the difference of rear drums over rear discs on a Zero, and probably a lot of kit cars.
The brakes used generally come from a donor vehicle, in this case a relatively heavy and servo assisted sierra, the brakes are balanced for that set up, we now put them into a 600Kg car (approx) and remove the servo, on disc/disc the efficiency balance will remain about the same, on a disc drum set-up, we have removed the servo, but have not removed a characteristic of drum brakes.
I was taught was "self servo action", but called "self-applying" on this Wiki site for those interested. https://en.wikipedia.org/wiki/Drum_brake
http://www.carhistory4u.com/the-last-100-years/parts-of-the-car/brakeshas a bit of history about brakes.
I am sure disc and drum debate will rage on, but drums will do exactly the same as discs by way of stopping you if designed for the right application.
However each has their advantages and disadvantages. Discs get rid of heat better (but can still fade if outside design), they are open to the elements, (discs corrode and brake dust gets on our nice shiny wheels) they also allow the handbrake to come off (another story), and contrary to the prices charged are cheaper to manufacture and fit in production, (could that be why they are fitted to basic family cars?).
Drum brakes are enclosed, (less prone to contamination and deterioration), are more costly to produce, require more time to inspect (higher service charges), retain heat, can fade if outside design capabilities.
So are discs better than drums? or are we sold that discs are better than drums? No opinion really, will put on tin hat and retire to my dugout, expect incoming :-)