I purchased a new home built in 2007 and the builder is no longer in business. It has a thermostatic shower mixer valve with 3 volume valves above it for the 3 shower heads. It may be a Hansgrohe valve (but I'm not sure). Everything works but I've discovered that the mixer valve has a significant cross-leak between the hot & cold supply.

I've tested this by leaving all 3 shower valves off and turning off the home's main hot supply. With the hot supply off, the other hot faucets in the home will still run at about 50% of the cold faucet rate. Turning the mixer knob changes the "leak rate" (but it doesn't stop it) and I can hear the water flowing inside the mixer valve.

I pulled off the trim and I could post a photo of the valve if needed. It has two 16mm hex caps (left and right) with screw-slot adjustments centered on them.

Any advice would be greatly appreciated.