shower control doesn't shut water off

i have a price pfister 808-A750 single control shower valve that doesn't shut off the water. when it was installed a year ago it worked. now water pours out of the head even when the handle is in the off position. i took the chrome cover off, and unscrewed the plastic cover that holds the stem with the metal plate against the plastic mixer. the springs and seats and o-rings in the plastic mixer look fine. the circular metal plate on the back of the stem that the handle attaches to looks fine, and it appears to line up correctly in the off position so that no water should be coming out. but it does. what's going on????

jdisston said:

. the springs and seats and o-rings in the plastic mixer look fine.

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How do you know?
All it takes is very small nic in the washers to cause the leak. Replace the springs and washers first, whether they look good or not.