In order to add an Inhibitor to my houses central heating system, I've
attempted to drain the rads using a drain off cock near to the boiler
on the ground floor, opening radiator vents on the first floor. That
worked OK ( air being sucked into the vents ) so I tried the ground
floor rads but they continued to have +ve pressure with water dribbling
out of the vents.

This was still the case even though the water stopped flowing from the
drain off cock - but the radiator vents are about a foot above it.

I'm probably making a beginers mistake but can't understand why these
rads didn't also at least partially empty through the drain off cock.

I had stopped water refilling the header tank using it's input stop
cock in the loft.

Eventually I gave up and finished the job without draining the ground
floor radiators which I hope was OK given I was just trying to get the
inhibitor into the system. But I would like to be able to drain the
whole system for future maintenance jobs.
Thanks in advance for suggestions.

Phil