I have old cast iron radiators that are in great shape. My house is about 118 years old and has three levels. The radiators were probably installed sometime in the 20's or 30's. A new boiler, a Raypac Type H residential boiler system, was installed in 1988. I'm pretty sure I know how to fill and bleed the system. In fact, I filled and bled the system over the summer after our construction project was completed. I couldn't get the pressure to reach the suggested 12psi. I got it to about 5psi but it was summer, so I didn't worry about it. Over the next several months, I checked the pressure from time to time to see if it has fluctuated, and the pressure remained the same.......until I turned it on a couple of months ago. The pressure went to zero, so I filled and bled the radiators again. Once again, I wasn't able to get the pressure high enough, and in a few days, the pressure went to 0 again. My questions are:

1) Am I filling the radiators the correct way? Once I fill and bleed the radiators, I turn the water off right away. Should I keep the water on until the pressure builds?

2) I read in a website that I need to pull the pressure relief on the boiler to make sure the boiler is full. Is this correct? If so, what does a pressure relief look like?

3) I went on the Raypac website, and it says "Open valves for normal system operation, fill system through feed pressure
regulator to minimum 12 PSI. Manually open air vent on
the compression tank until water appears, then close
vent." Huh?!! Compression tank? Can someone explain in layman terms?

4) I heard that it is normal to fill and bleed the system a couple of times to get all of the air bubbbles out. Is this true?

Thanks for your help!