Backflow vent leaking.

Weil McLain Gold

9D-M3 Back flow

Watts pressure regulator 12-15

Turned on heat for the year, lots of air in second story baseboard so I went down to look at the boiler pressure was low almost on zero so I manually added water pressure regulator. 9D-M3 started leaking from the vent afterwards, seeing it was old anyways I just replaced it. Before installing new I did flush out the inlet supply line as to not run any sediment through new back flow. Now back flow still leaks but I believe it is only leaking when the boiler cycles? Pressure on gauge is 14ish at 200° obviously drops as temperature cools off have also have to add water as the air has been working its way out. Any ideas? Thanks!

Edit one thing to add, I know water pressure to the house is above 12-15 psi as I have a pump that pulls it up from the city gauge reads 55psi on that tank.