shut off the water at the PRV - shut of the boiler - drain the tank - then close teh valve at the tank - check in the house for a bleeder on the fin tube radiation on a elbow, or is it a cast iron system where a vent could be ontop of the radiator...with the system closed up and teh pump turned off open the valve at the highest point in the house - open the water supply at the PRV - vent ti tuntil water comes out - close the vent start teh pump - shut the pump off after 10 minutes and vent again - cycle this until the air is out of system - thhis way the expansion tank will have enough air voulme and water to capturee any air left in the system and expand when the water is heated - it is normal for the water pressure to rise to 25lbs at 180 degrees you should have a 30lbs saftey on the boiler.

if the pressure keeps kreeping up - drain a little pressure of the system - keep the make up water valve closed if system does not build up pressure after running it for a day - your PRV is leaking by / shot...not even worth playing with it - go to HOmedepot or LOwes or a a shop - if you have to change PRV you should change the saftey too, youll propably have to drain the system anyway - do it all.