sand filter leaking

Welcome to TFP!

If water is coming out of the backwash port when the valve is set to filter, there is something wrong with your main valve. On a multi-port valve the most likely problem is the spider gasket. On a push pull valve the problem would usually be an o-ring inside the valve.
 
Or you could be like me and put the handle on backwards when I replaced the spider gasket :hammer:
 
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