If the windshield washer continues to spray continuously, there are two reasons. Either you've connected the hoses incorrectly, so that the air pressure continually pushes the valve upwards, or the valve is broken.
To make sure you've connected the hoses correctly, check that the water tank hose is connected to the straightest fitting and to the fitting that ends with a 45-degree section of the hose at the washer head.
If the washer stops spontaneously, or if you discover a leak, you'll need to replace the valve - repair is not possible in most cases. You can buy a completely new switch for this, but if everything still works fine and you're still happy with your existing switch, you'll simply have to replace the valve.
This valve is only provided if the washer is on the steering wheel; if it's mounted on the dashboard, then repair is not an option and you can only renew the complete switch.
Note : Carefully dismantle the valve and reuse the pressure pin. The pressure pin is not supplied with the new valve.
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