I'm not sure you can test those MPS-U45 to be *good*. You can test them and get a result if they are bad, but just because the readings look right doesn't mean they are working properly. Best to replace that one if you want to rule it out of the question. If you don't have a spare to replace it with, let me know and I'll mail over a CEN-U45.

I am about 82% sure that the wiregate (which I don't have) may be the issue.  It seems that the one in yours (pictured) with the 90deg bend is completely unique to BH.  Nobody seems to know the part number.  If you manually move the plunger by hand (with the ball in trough) can you just check for me if the wiregate touches the ball at all.  It was suggested that it was designed to do this?

Basically on mine, If the wiregate stopped it kicking back that bit I reckon it would be 100%....

I've emailed PBR to see if they have that part - but without the part number I don't like my chances.