Great question — answering as the UphillTech team:
Mechanical For a purely mechanical solution, you may be able to use a pressure relief valve , though this requires the pressure be high enough to trigger it. These are fairly common parts, and a variation called a temperature and pressure (T&P) relief vale is installed on hot water tanks. You can get valves with varying pressure ratings/ranges. I believe when operating near their set point, they do allow some amount of water through (eg, it is not a binary on/off) so you'd have to consider if this is acceptable for your use case (eg, if the pressure drops enough during normal usage that it doesn't normally let water by and if it's acceptable that some does go by). I think this would really...
Full answer by gregmac: https://diy.stackexchange.com/a/110011 (CC BY-SA 3.0).
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