I had the very same problem with my X3 recently and it was really easy to resolve.
At the front of the screen bottle, there are two pumps (one for front and one for the rear). They simply click forward and then lift out revealing a rubber stopper which the pump sits in.
Simply remove the rubber stoppers and attached to them is a filter which gets clogged up. Clean the filters and simply put them back in.
One thing to note. When you remove the filter, you will drain the entire content of your screen wash bottle!
Sunday, January 25th, 2009 AT 4:00 PM