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Flow checks don't cause over pressure problems. Since you've replaced the feed valve the only other likely culprit is a failed expansion tank. Or if it's a 50 year old expansion tank hung up between the floor joists it water logged and needs to be drained.
Since the pressure " always returns to 12 PSI when the boiler cools" this indicates this is an expansion problem. If you have a new expansion tank it may be too small. I need to know the btu input of your boiler and the type of radiation (copper baseboard, cast iron baseboard, cast iron radiators, convectors) you have. Like I said before a failed flow check can't have any impact on system pressure; flow yes, pressure no. I wouldn't go through the trouble of replacing the flow check in this case if your intention is to fix a pressure problem.
A clogged fitting is possible. That said you can't rule out the expansion tank until you answer my questions related to sizing. It doesn't matter if that size tank worked before.
Ok, that's a water heater expansion tank it's not for a heating system. An Extrol 30 is the right one for heating systems and is the common size for an average system. I've never used a water heater expansion tank on a heating system but the principle is the same. You could try dropping the charge to 12 psi but you must do that with no pressure on the system.
Yes, you can leave the tank in place and bleed off the excessive pressure. You don't have to completely drain the system just drop the pressure to 0.
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