I have a Hayward DE-4800 and the backwash valve started leaking last night (i.e., it won't close all the way and water is still draining through the backwash tube). How do I fix it, and what part(s) do I need?
You shut down the pump, then remove the screws around the perimeter of the top of the valve assembly. Then lift the valve out of the housing. On top you will see the big, round gasket, with "legs" that seal between each opening of the valve inlets and outlets. One or more of the legs on yours have deteriorated and are leaking. You just drop the new gasket in place, put the valve back in and reinstall the screws.
One thing to be careful of, if the screws just thread into plastic... You want to get the threads of each screw into the existing threads in the plastic BEFORE you start tightening the screws. Otherwise the self-tapping screws will start new threads. This weakens the plastic and eventually the screws won't hold in place any more and you'll get to buy a new valve. You do this by gently turning the screw backwards while it's in the hole, as if you are loosening the screw. Keep turning until you feel the starter thread on the screw click into the thread in the plastic. Now you can tighten the screw and it will go into the threads that it tapped the first time it was installed.
If the valve body has metal threaded inserts for the screws to go into, then you don't have to worry about that step.