Problem with Eheim Filter, please help

Has anyone else experienced this problem:

My Eheim filter is stuck on full blast, it no longer fills and
drains,
fills and drains...it now is permanantely full and it keeps pumping
steadily without the lull between.


It seems that the problem is the float valve is stuck in the up
position, it no longer sinks. Can
anyone suggest what might be going wrong with it?


I have taken it apart and made sure all seals are good, and it is all
clean inside.


Any help appreciated.