high pressurs?

It will not go over 3.5, as the over pressure valve has opened, You say you have drained it, you must still have the filling loop open, only way it can keep filling by itself, or filling loop valve faulty. If it is you need to turn stop cock for water flow off. before more damage occures, you may find when it is sorted, that the over pressure valve will not seal fully, and you will continue to loose pressure via it, so it then need to be replaced.

SD
December 2011