My Kohler Kitchen Fauset K-10412 Has slowed in water flow to about 40% of it's original flow, whether on cold or hot. Will the valve kit fix it? Should I replace any other parts?

Try to clean the Aerator first see if it will help, open the link below and see the image, Thanks.

John

Kohler Plumbing

It was simply the Aerator. I can't believe I forgot to check it. Thanks John.

Thanks for the update and am glad that we could help.

Regards,
John