leaking TPR- changed still leaking

I have a gas boiler hot water baseboard heating system along with a gas hot water heater. The home is about 8 years old. Recently the TPR valve started leaking intermittently on the hot water heater ( not excessively hot water so I don't think its a thermostat problem). I have a water pressure regulator on the main line from the city supply ( just background). The pressure is good and I actually dropped it a little...I changed the TPR and it still leaks a little. I do have an expansion tank near the boiler and it sounds hollow and when I let a little air out of the refill air valve only air came out ( seems OK ?). Do I need another smaller sized expansion tank before the hot water heater? I'm out of ideas. Seems strange that I might need another expansion inline after 8 years? I have two heat zones w/circulating pumps and added another in the basement this summer, but that was just tee'd off the the other two? Any help would be appreciated!
Joe

A boiler system with a PRV requires an expansion tank on BOTh the inlet to the WH and one in the boiler circuit. If you only have one, and it is in the boiler circuit, it is isolated from the potable water system, and won't do anything for expansion when using hot water in the house. Add one to the potable water system, on the cold side of the WH.

Could you be a little more specific on the plumbing from the city water regulator to the boiler and hot water heater. And just where in this is the existing expansion tank. It is not clear if the tank is on the boiler circulating fluid or on the main water line. You will need an expansion tank on the household cold water line because the pressure reduction valve from the city water effectively makes you house a closed system.

You might drain the water out of the expansion tank and properly set the air pressure in the bladder (if there is one). If the tank does not have a bladder you may have "used up" the air in the tank and it needs to be redone.

Have you by any chance recently opened the feed water valve for the boiler and left it open? Do you know what pressure you have in the boiler output piping (if you have a valve open and it is not properly piped with a working backflow preventer) and the main house cold water line? The boiler pressure should not be anywhere near enough to trip the relief valve on pressure. You should have a gauge in the boiler. For the house you could install one on a T or buy one that attaches to a hose faucet.

A failure in the pressure reducing valve can generate excessive pressure.

Thanks for the replies and advice. I did put a tee into the cold water inlet before the water heater and installed another expansion tank. Well ( knock on wood), that was yesterday and so far the problem seems resolved. Thanks again.