Assuming the 1-way valve was okay at the start of the flight, when the 1-way check valve fails and blocks the crankcase pressure (fails closed), the pressure in the tank will drop as the fuel is consumed.
But, if the 1-way was never working to begin with, then the tank is at ambiant pressure at the start of the flight, and the engine keeps sucking fuel out and builds a vacuum in the tank.
Either way, the needle settings will go from "ok" to "too lean" as the pressure drops off and the vacuum builds.
If it fails "open", then it is like a direct connection from the backplate nipple to the tank's vent nipple. This could flood the engine's crankcase with raw fuel being sucked in from the tank on the piston's up-stroke.
This is not as bad as the "failed close". At least the tank is vented and not building a vacuum. But, you may not realize the 1-way is failed open - you just readjust the needles because suddenly it is running a little leaner than it used to.
The OS and the YS 1-way check valves are the same and interchangeable.
Keep the line from the backplate to the 1-way short - no more than 1". And use stiffer than "normal" fuel tubing - flexibility is an enemy.
Blow through the 1-way - it will open and will let air flow in 1 direction, and block it in the other (it might take a lot of "mouth pressure" to get it to open).
Remember: the air flows from the backplate to the tank, so the 1-way has to be oriented correctly.
You know you are in good shape when, at the end of the flight, you open the vent line and you hear a big "whoosh" as the extra pressure in the tank is allowed to escape.
You get used to hearing that. If you don't hear it, then stop and figure out what is wrong.
No pressure in the tank at the end of the flight means trouble.
If you had the presence of mind to check that after your engine-out autorotation, you could have known if there was good tank pressure or not and where to look for the engine failure.
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