It appears that this boiler is serving one zone, which means that there is not a lot of piping. If the volume of water in the system is small, then it doesn't take as much water to change the pressure as a larger system. You don't indicate how you are determining when and how much water to add. Should we assume that you are watching a pressure/temperature gauge ("tridicator") on the boiler? what pressure range ? (should be 12-15 psi when cold)
I suspect that the expansion tank has failed. When a small amount of water leaks out, the pressure would drop much quicker when the expansion tank is water logged. Tap on the tank. The end attached to the boiler piping should sound dull and the opposite end should sound bright (hollow). If it is dull all over, it needs to be replaced.
In my experience, all of the boiler pressure feed regulators that I have replaced had failed in the closed position (no longer maintaining minimum boiler pressure. I have never heard of a feed regulator failing in the fully open position. What if you don't keep up with feeding water to the boiler and the pressure drops to zero? The boiler could start making steam, which can be dangerous. (You should be replacing the boiler relief valve about every 6-8 years.)
Frodo is right about the dielectric union. Copper in contact with iron creates galvanic corrosion. I don't see the need for a condensate pump. This is clearly a non-condensing boiler.
Adding water to the boiler system, especially if flushing the entire system, introduces more minerals and dissolved oxygen, which increase corrosion inside the system. Do not flush the system unless there is a specific need. If you were getting a lot of corrosion products when you have flushed, you need to pay attention to the quality of the feed water.