What do I do if water leaks from my kitchen spout while my side sprayer is in use?

The issues may be caused by a defective diverter valve/ assembly.
You may start with cleaning the diverter system by soaking the part in 50/50 vinegar and water solution for 2-4 hours to remove debris and mineral build-up. (use old toothbrushes)
If the problem continues to exist, then most likely issue is a defective diverter valve.
For example, the replacement diverter for faucet KB1791BLBS is KBTEE1791 (Part#24 and #25).

https://media.kingstonbrass.com/pdf/technical/parts/kingston-brass-KB1791BLBS-parts-diagram.pdf

To identify the replacement, you may visit the item page by searching the model number of the faucet.
Once you reach the item page, all the replacement parts should be listed under the “Replacement Parts” section at the bottom of the page.

If you are having difficulties identifying the part, please feel free to contact our support team for further instructions.
Tech support: [e-mail]