Pressure Relief Valve dripping

I have a Watts 374A Water Pressure Relief Valve that is dripping a little less than an ounce a day. I already replace the expansion tank. (The PRV was spitting out quite a bit more before that.)
Somewhere at the Watts Web site, I read that some dripping is OK - and means "that the valve is working." I could live with the water loss, if it was just water. But glycol is too expensive to just pour down the drain.
Can a PRV be serviced, cleaned, etc., or must they always be replaced?
My Watts valve has a set point of 30 PSI. I've observed that my hot water baseboard system is operating between 12 and 18 PSI. I've NEVER seen it above 18 PSI, so it seems like a 30 PSI valve shouldn't be releasing ANY water. What is normal?
Thanks for any insight.
thomas

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The valve should not drip if it is not seeing 30 PSI. Just because you've never witnessed your system above 18 PSI doesn't mean it doesn't spike at times due to thermal expansion. In order to condemn the relief valve you must confirm peak system pressure over a 24 hour period by using a test gauge suitable for that purpose. If the valve is found to be bad it must be replaced. It is not to be serviced in any way. If the system is seeing spikes above 30 PSI the problem is in the expansion system, including design, piping and fittings, and not limited to the tank itself.
FYI, the abbreviation "PRV" is most commonly used for Pressure Reducing Valve, something entirely different from Pressure Relief Valve.
MM