I have a Kohler single lever style shower faucet. It started leaking so I purchased NEW from Home Depot the MIXER CAP that is behind the lever. I replaced it and the leak persisted. I contacted Kohler and they sent me a new MIXER CAP for no charge and also sent an "O" ring that goes behind it. I replaced both and the faucet still leaked. I called Kohler back and they said that it would need to have the PRESSURE REGULATOR behind the MIXER CAP replaced. They sent that and I installed it and the faucet still leaks. I've had 2 MIXER CAPS and a PRESSURE REGULATOR replaced and still no repair. Any ideas? Anyone?

The balance spool would have no effect on correcting a drip. There should be at least two o rings used in repairing the drip.

You must not be repairing it properly. Most of the time the pressure balancer is the cause of the leak and replacing it stops the leak. You have replaced all the moving parts, and if installed properly that would normally stop any leaks.

quote; The balance spool would have no effect on correcting a drip

With a Kohler faucet, that is often the only cause of a drip. It is riveted together and the seam can start to leak. If you remove a Kohler spool and there is white "debris" along the seam, that is where the leak is.

All the research says it is the Pressure Balancer and I concur.
What I can't figure out is how one could install it incorrectly.
It doesn't seem like it could go in there wrong.

Any thoughts on that?

IT depends on which one it is. One version has to be inserted in a particular way, and then rotated into position. If not done that way the seals can be damaged.