Lomcevak - I took a number of windscreen wash valves apart under F760 investigations raised because of the problem you and many others encountered. It was an odd concept, effectively a pneumatic timer relying on a stored volume of air escaping from a reservoir through a restrictor to close off the valve after 10 seconds. Unfortunately, for reasons best known to himself many years previously, the valve designer in the Brough drawing office had chosen to size the O ring groove dimensions to tolerances which were different from those recommended by the O ring manufacturer. Nothing was ever done to improve the design, Nowadays I doubt any aircraft manufacturer would consider using a pneumatic timer valve when a microchip and solenoid valve could do the job, but that's progress for you.