A pressure reducing valve, sometimes called a pressure regulator is used to regulate the incoming water pressure to a building.  Often, the incoming water pressure coming from the city water department will be excessively high for some areas, and the use of pressure reducing valve on the incoming water mains is the mandatory solution.  For most areas, the acceptable water pressure to a building is 80 psi or lower.  Anything higher than 80 psi is considered excessive and potentially dangerous to the plumbing system, and must be regulated and reduced via the pressure reducing valve.  Over time, a pressure reducing valve will naturally wear out and begin to malfunction.  They can break and let all the water pressure through, instead of reducing it.  They can start to make noises and chatter or whine as the valve struggles to stay closed.  Sometimes they can become clogged with sediment and reduce the water flow to a trickle.  They can even corrode out sometimes and begin leaking as with any other water pipe valve or fitting.  These are some of the many reasons you will have to replace your old pressure regulator.  The most common sizes for a residential building is a ?