does someone make a hydraulic brake for...
does someone make a hydraulic brake for rims on a bike
If your asking about rim brakes specifically, then no. To my knowledge there are no hydro rim brakes. Nothing else compares to hydro discs. Not only because of modulation but also stopping power and performance in adverse conditions such as mud or water.
They do make them for trials bikes. they are hard to find in the us an they are notoriously hard to adjust and expensive. Probably better just to stick to discs.
Many moons ago Magura made a hydro v-brake. It was a neon green set up. Had a friend who had them and had nothing but problems with them, but they do/did exist