Mine get covered constantly, hence testing the line outside of the tank idea. Did a water change today and checked on the valve and can tell its already starting to lose its effectiveness.
For the little check valve i was using it has gathered some condensation inside and a bit of algae. Will do a wrap of black tape on it to see if that helps. Doubt it though

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As far as check valves failing, when u have 900 gallons an hour passing through even the smallest pieces of debris will eventually stick to the innards of the valve. Any mechanical movement will eventually have some sort of interference lessening its effectiveness and eventually stopping it from functioning.
I recall an old gravel vac I had was built with a mechanical check valve. The sort that had a flap that would cover the outlet when current began moving in the wrong direction. Basically flow would pull a flap down and the water would stop moving. This however doesnt stop the siphon it just stops the flow. The siphon will hold until air is introduced into the line. Thankfully when the valve stopped working on the gravel vac it just would pull back the dirty water in the line.
Not to say that a check valve inline wouldnt work atleast for a bit, but the same way that debris blocked my gravel vacs valve the same thing would eventually happen to the inline.