A lot of people have problems with the CPR overflows in that air bubbles eventually build up in the u-tubes and affects the performance of the overflow or could completely make it lose siphon.
In the end, some people with CPR overflows use an "Aqua-Lifter" pump to pump out any air that builds up in the u-tube. They are about $20 + shipping from MOPS.ca (I also have one for sale
)