I recently had to give up running and decided to start commuting to/from work (about 15 miles round trip) so I could maintain my cardio fitness. One problem is I like to ride my cruiser (a Masi Soulville) and use that as my commuter. It's a coaster-brake-only setup, and I was wondering how I could tell if it would accept a front brake, and if so, what to look for in said front brake.
To install a front brake you will need to have a fork that will accept the brake caliper. There are three potential options for this: A standard road caliper (hole through the fork crown), cantilever/ v-brakes ( one post on either side of the fork blade) or a disc mount (bracket at the end of the fork on the non-drive side). The Masi Soulville should have the first option, a hole drilled through the fork crown for installing a standard road caliper. You will need to get a road caliper that is intended for use with a flat bar brake lever, as many road calipers are intended for use with drop handlebar levers.
The thing you have to figure out is the "brake reach" or distance from the hole in the fork to the rim of the tire.
As with most bike things Sheldon has answered it better than we ever can.
My guess is that you'll need a brake with a very long reach for a cruiser bike. Oddly enough these are generally dirt cheap.
The other gotcha is that they don't brake all that well due to the flex in the long arms, but they are certainly better than no brake at all. With the relatively fat tires and slow speed typical of cruiser bikes, they work well enough.