Did you try stopping the impellor from starting the wrong way yet?, does it flip back or not?...
It would be nice to build a mini reservoir and have little rubber flaps that covers each inlet, the pressure would either flip them open for the main flow, or close down for the bad one, they could be very pliable as well so they'd open easy enough without restricting, the system back pressure of the water itself would hold it shut from flowing the wrong way, bit like how a bicycle pump works, so there'd be no positive pressure valve to restrict the flow, they could both open up as well if there was'nt too much backpressure...
I'm pretty sure there are no~return valves that work off of the back pressure rather than having a spring involved, a 'passive no~return valve' maybe?. it's just a job of getting the size/flow~through of the valve right...
RS is a good place to start I 'spose
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