I know i have had the same prob and posted here. You are on the ball with fixing the problem.
Also you could just leave the pressur relif valve open over night and try again in the morning.
EDIT:
That is if you want to try a beer in the mornng.
Merry Xmas
Just purge the gas from the headspace of the keg every hour or two. Once the gas is purged from the headspace, CO2 will come out of solution from the beer to equalise the pressure (Boyles law or something isn't it?) and over time the beer will reduce in carbonation...