Hot Water Boiler Question

I have a hot water boiler system that is one zone. There are 3 supplies and 3 return pipes. One section of my house that are on the same supply run have always had issues providing heat. My house was built in 1940 with multiple additions added throughout the years. I am renovating a bedroom and replaced the baseboard because a lot of the fins were damaged. During this process I went up in the attic and found 2 valves on the same run that were closed. I assumed these were the supply and return and could be the issue on why the heat wasnt working well back there.

I opened up the return and supply in the floor and immediately heard water flow through the pipes. My issue now is the pressure reducing valve keeps feeding the boiler water and hasn't shut off. It probably ran for over an hour before I had enough and shut off the water going to the boiler all together (the boiler is not on). My main question is, what would cause the pressure reducing valve to continuously feed water (it was replaced last year) besides a leak in my system? It just seems that since the pipes are above my ceiling and I let the water fill for over an hour that I would have notices a leak by now.

Thanks in advance!