You may have a knackered TMV on the mixer. If they still make the replacements then swap it out. Which you know.
Take it out and show it to a plumbers merchant to see if they recognise it or Google any part numbers on it.
Put check valves on the hot and cold. Stops water going the opposite way.
If the mixer is faulty non return values won’t make a diffrnece.
Having never worked on a shower like that yet and never seeing anything like that in the merchants. I think you’ll struggle to get a replacement, probs best online.
@konastab01 started
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You may have a knackered TMV on the mixer. If they still make the replacements then swap it out. Which you know.
Take it out and show it to a plumbers merchant to see if they recognise it or Google any part numbers on it.
Put check valves on the hot and cold. Stops water going the opposite way.