It sounds as though the siphon washer is torn. When you pull the handle it lifts the siphon piston which has a thin plastic membrane at the base called a diaphragm washer, usually just referred to as a siphon washer. They are usually rectangular or D-shaped and are available from all good DIY or plumbers merchants. You'll only tell which by looking at the siphon piston.
If you have a 2-part siphon you don't need to remove the complete siphon, just the housing with the piston in.
Nick from sunny Sale, March 2009
