Brakes for a mountain bike frame (26'r) with 27" road wheels I have converted a mountain bike to a commuter. the frame accommodated 27 " wheels, so I installed for a better ride. I have a road carbon kinesis fork up front with a v-brake, but need a rear brakeset. 

The boss positions for the rear brakes, however, are too low on the seat stays for a standard set of v brakes to properly contact the rim. 

Is there a brake that is long enough to contact a 27" rim when installed on a 26" mountain bike frame?
I have not tried, but at Paul Components say these Moto BMX brakes might work with 700c rims on 26" frames. But they might turn out to be too expensive for your purpose.