We are sorry if some of the system maintenance requirements were not clear over the years. The cartridge instructions have stated for 5+ years that once the cartridge is installed, it should soak in the water with minimal or no water flow for about one hour. We do not recommend flushing for one hour as that is quite the opposite of what the cartridge needs in order to activate. Once the cartridge is installed in the system, water should be returned back to the system until the canister has been filled. At that point, the system should be bypassed so the cartridge can soak and activate in that water for about one hour without flowing water. This can be accomplished by bypassing the system itself while it soaks. After that time, the cartridge media has fully activated and is ready for use and the bypass can be turned off allowing the water to flow through the system and the home.
As far as the O-ring on the system housing, that does not need to be replaced every time a cartridge is replaced but it should be replaced about every other cartridge change to ensure a good seal and prevent leaks due to a worn out O-ring.