druzac said: ...Inside of tank bladder appears to be in good shape.
Looks can be deceiving....chances are the float is hanging on the bladder when it gets to 1/2 tank. Since you've already replaced the gauge, sender, and circuit board, I would expect the problem is none of those things. If you take the sender out of the tank, you should be able to move the float by hand, and watch the gauge to see if it responds accordingly. If it does, then the problem is the float hanging on the bladder.
The pump will run for a few seconds when you do this, but it won't be a problem.
Someone here not too long ago had a similar problem, and they just bent the float arm enough to eliminate the hanging up problem.....might be something you may want to do, also.
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