Replacing leaky wheel cylinders is a must when doing brakes.  Peel the edge of the rubber back on your old cylinder.  If it is moist, replace both sides on that same axle.  It's probably a good idea to replace all four at the same time while you have the brakes apart, but it's important to do at least the fronts or rears together.  Never change just one.

Genuine German replacement part.

Your brakes could save your life!!!

NOTE: Photo is for website ilustration purposes only.  Your particular cylinder may be different that what the photo looks like.