Guest bathroom combination shower & tub. Guests complained there is not enough hot water when showering. I checked and it is true. Hot water flows from the sink hot faucet in abundance, same with all the other faucets in the house. I removed the single handle and adjusted the red stop on the valve stem, no avial. I removed the O-ring and stop mechanism from the stem and turned the stem as far as it would go. Hot water, yes, as hot as the rest, but little volume. I hope there is not solder in the hot side pipe joint. Any suggestions? Kohler 500520.

Does the valve have integral stops?