I'd like ideas for a rear brake system for a small trike (see http://www.carcentric.com/YamahaSR250Trike.htm for the trike plan) with these features:
1) it would have a rotor which could be mounted directly to the Comet differential case so it would affect both axles,
2) it would be mechanically actuated (not requiring a hydraulic master cylinder, lines, or fluid) by the Yamaha's rear brake pedal,
3) it could bring 750 lbs of people and trike to rest within a reasonable distance at speeds up to 45 mph, and
4) not cost a great deal (nothing exotic, preferably using common parts).

My thoughts so far involve using the rear brake from a Porsche 914 because it's emergency brake portion is cable-operated, yet its lever acts on the caliper piston (there is no separate friction material for the parking brake).

There are plenty of rear calipers with built in handbrake mechanisms. The Ford Sierra/Granada/Scorpio caliper is popular but there are plenty of these fitted on other cars.

Pretty much all modern VWs and Fiats have them. 38mm piston diameter is the most popular, Sierra/Granada/Scorpio is a 43mm piston.

You could use any motorbike caliper that fits and then use a after market spot caliper for the hand/parkbrake.

I have seen trikes with mountain bike calipers as a parking brake, rally design also do a cable caliper for about £40