Isi cream whipper head valve sticks open

I have an isi 1/2 liter cream whipper which I purchased in 2010. It has worked well until recently when the head valve began sticking open. I purchased a new head valve, but the problem remains. It has been thoroughly cleaned and also has a new lid gasket.

I tried to use it today for the first time with the new valve. When I first dispensed some cream, it seemed fine, but when I released the lever, cream continued to ooze out. The valve was not returning to the closed position. This is what it was doing with the old valve. Any suggestions?