My commuter is a '76 fuji track, undrilled so I run a front kierin brake. My "cafe" bike is a '79 raleigh track, on which I run a rear kierin brake. I wanted to keep the front clean looking, but still have some sort of brake option. The rear brake is useful as I can stop quicker using it + my legs than with my legs alone. Though not as quick as with a front brake. It is also useful on steep descents when I don't feel like fighting to stay in control the whole way down with my legs alone.
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1989 Schwinn Paramount OS
1980 Mclean/Silk Hope Sport Touring
1983 Bianchi pista
1976 Fuji Feather track
1979 raleigh track
"I've consulted my sources and I'm pretty sure your derailleur does not exist"