Silicone does not stick too good to some types of plastic. If the overflow is made of one of these plastics the water pressure on the outside of the overflow can cause the silcone to seperate from it over time causing a leak. You can make the joint stronger by running a bead of caulking down the inside of the overflow. That way the outside water pressure is pushing against the bead on the inside as well.
If you put a stand pipe on the inside as Brad said you will equalize the water pressure on both sides of the overflow.
Make sure your return line has a siphon break in it to stop back siphoning as well
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