I've been busy at work on projects and couldn't visit the forum as much as I would like to.
Now that the check valve is in place and some pipe replaced, have you tried a thorough flushing of the pipes and the pump?
I'm not confident that by adding another check valve where you indicated would do much better, but if you have the time and the money, you could give it a shot in the same manner that you already piped below the sink.
Filling up the sink basin with a good amount of water, adding a citrus type cleanser and letting the pump flush out this lines may give you some results if there is build up from years of use. Even a treatment of the drain that the discharge pipe connects to probably wouldn't hurt.
Choose your products carefully, though, if your waste goes into a septic tank.