Classic & Vintage - Odd Brake Caliper Placement on old Trek?

I have a friend who has an old Trek 800 series mountain bike (maybe from the '80's?) and it has a set of brake calipers on the rear wheel that are mounted under the chain-stay! Some of you have probably seen this brake design before but I never have. Does anyone have any information about this brake design? It seems a bit odd to me and my friend says that it's a pain in the neck when he rides through mud or snow - the caliper tends to collect the aforementioned like a beaver dam.

They take a special brake called a "U" brake that that is different from both cantilevers and V brakes. The bosses are in a different place. The theory with the placement was the chain stays are bigger and stronger than the seat stays. This would give you less brake flex. The problem with them is because of the location they filled up with mud and you ended up with no brakes. Roger

Just about every mountain bike built in 1987 had them. Those bosses will also take a roller cam brake. I have a Japanese-made Peugeot with the rear brake under the chainstays. It had a Suntour roller cam, but I switched to a Capagnolo Euclid U brake. The brake won't get clogged so badly if you run a fender all the way through it.

http://inlinethumb30.webshots.com/40861/1315592111068014369S600x600Q85.jpg (http://sports.webshots.com/photo/1315592111068014369YdAZEe)

As mentioned, they are U brakes.
Common from about 87-89.

gack....bike brakes from hell. Clog with mud....they attracted mud like Trailer parks and Tornados

My mountain bike (see signature) has an under-chainstay U-brake and a RollerCam in front. Yes, the rear brake does tend to pick up dirt, but the brakes provide me plenty of easily modulated stopping power. Yes, I would have preferred a more conventional location for the rear brake, but the system I have is not bad.

There's is another reason for the lack of flex of the U-brake. The mounting studs are are above the rim, as opposed to below it, as in cantilevers or V-brakes. With the studs farther up the stays and closer to the bridge, the stays are more resistant to outward flex in response to braking force.

There was another advantage to the U-brake. Early ATBs had very long chainstays. By the late 1980s, these had shortened considerably and the geometry become more agressive in general. With ther shorter stays, some riders would contact the cantilvers with their heels, dislodging the yoke cable and resulting in no brake. The narrow profile of the U-brake prevented this.

U-brakes are still common on BMX bicycles.

My 1988 GT Karakoram had the best of both worlds. It had a rear U-brake that was mounted on the seat stays.