In almost all cases the "spool" valve is bad. It allows the pump to keep running even when you don't have the trigger on and spraying. Where would all the water go, if not back around the pump. Not too hard to find on most models. It's sort of like a reversed check valve.

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DS
2010 CL550 - Heaven help me but it's beautiful
87 300D a labor of love
11 GLK 350 So far, so good
08 E350 4matic, Love it.
99 E320 too rusted, sold
87 260E Donated to Newgate School
www.Newgateschool.org - check it out.
12 Ford Escape, sold, forgotten
87 300D, sold, what a mistake
06 Passat 2.0T, PITA, sold

Las Vegas NV