How do I fix my leaking pressure control valve on my furnace?

I have an older home heated by a radiator system. I just had the boiler inspected and cleaned by a pro. At the time the pressure control valve was leaking (about 1 drop every ten seconds). He recommended replacing the valve. I purchased anew valve from a plumbing shop and installed it myself. The problem persists. I don't think the new valve is bad. I suspect the pressure problem is elsewhere in the system. I have checked the equalizer tank. It is empty. What else could it be?