pressure keeps dropping?

you certainly have a leak of some discription, have you checked around the radiators ? for leaks at the bottom of the valves try feeling the carpet where the pipe goes through the floor also check the the pressure release valve is shut off properly you can check to see if there is water dripping out of the blow off (copper) pipe comming out of the wall if there is water dripping out of there then the pressure relaease valve is letting by or its just not seated properely try giving it a quick flick.

DIY Dave
February 2011