The problem seems to be either dirt/debris in the valve body or on the diaphram. carefully take the questionable valve apart and look inside for small debris that can be removed (sometimes a small pebble or rock will cause the diaphram not to reseat). Gently flush the opened valve body by cracking open the water main (best done in a 2 person operation with 2 way communication). Don't go to much or you're likely to blast material back into it. Take a clean towel and wash and wipe the diaphram clean (inspect it for any cracks or tears). Rebuild the valve. If that does not work, try a replacement diaphram... If that doesn't work, consider a warped valve body and change out the entire valve.
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