My ER-25 chuck has the same problem. At the top of the mill there is a tubular nut with flats on it that the drawbar jacks against when releasing. I used to do that nut up as tight as I could and hold it with a wrench when tightening/loosening the collet nut.
That was OK when tightening the collet nut of course. As with all my ER chucks, I don't overdo the chuck tightening – just a little nip – nonetheless at intervals when loosening the collet nut, the top nut would loosen and I'd have to remove the whole chuck and put it in the vice with soft-jaws to undo it.
Then I bought an ER-32 chuck which had proper flats on it which I prefer anyway since the size range suits me better and I rarely use the ER-25 any more.
Edited By Sid Herbage on 30/05/2013 02:08:40