Take the faucet apart and soak all the pieces in an Oxyclean solution. Re-assemble and see if it still leaks. The reason could be some build up of gunk on the seal. If the seal that closes the flow of beer is visibly bad, there is another one in the faucet that it can be switched with. The two are interchangeable.
You should also try Perlicks web site for additional cleaning information.
EDIT: I should say the o-rings are interchangeable on the 525, not sure about the other models.