So I have a purist rainfall shower and hand held. One or the other, whichever the diverter is on has developed a slow leak.

I replaced the valve with a new cartridge. Still leaking. Plumber says the problem is the diverter valve.

This doesn't make sense to me. The diverter just moves the water from the handheld or rainfall or both.

This is the part I replaced below. Although I only replaced the valve on the left and not the pressure balancing unit on the right.

Is the plumber right in that the diverter valve needs to be changed out?

Maybe I need to change out the pbu as well?

Call Kohler they have excellent customer service.

You more than likely need to replace the other part as well

Did you land on a fix for this? I am having a similar issue on a newer installation.

Yes
So originally I replaced the valve, the thing on the left above, and it didn't solve the issue, apparently it's the pressure balancing unit that craps out. So get both and change them out at the same time.

Kohler will actually send you a free replacement if you call them, but their shipping is slow and mine was constantly dripping.

Thanks!

How old was your install? I've heard of a valve needing replacement, but after a few years.

Spring of 2018

We have really hard water, probably need to think about getting a softener

That makes sense. Mine is only 4-5 months old.