How often should I expect a pressure regulator to fail. It appears mine has failed after about 4 years as it regulates pressure to 50 psi only if I open a faucet and let about 1/4 gallon per minute come out. Apparently the regulator leaks that amount which builds the pressure up on the outlet side. I bought a new pressure regulator and it had the same problem, even though it was new.

Not sure I understand your post but it sounds like your have a flow restriction somewhere not pressure problem