most brakes are simple screws that drive a pin against the rim of the table or against the spindle to keep it from rotating. The best ones have cast bases and dense wood or composite tops.
Unfortunately they are very expensive. One alternative that many growers here have been using is to use a potter's banding wheel, like the one manufactured by Lockerbie Manufacturing shown below. I have one of these and it can support a lot of weight and turns easily. Heck. They are designed to turn pots.
The only disadvantage is that the center of gravity is a little high. They're designed for getting the work up a bit higher, because the pot is the work, whereas ours is about a foot higher.
http://www.grant-ent.com/6lb3_04.htm
Regards,
Matt