Stuart Turner negative head pump runs continuously

[jamesBen80]

I've got a 2 bar negative head Stuart Turner pump, that just runs continuously. I've checked the pipe work and shower valve for any leaks and all is good. fitted a new circuit board, new pressure switch and 2 new non return valves. I've also removed and tested the flow switches with a magnet and mustimeter and both are operating as they should, but it still just runs as soon as you turn on the feed from the fused spur? The pumps been taken apart cleaned and put back together and all ok. I have checked the pressure in the little expansion vessel and its set at 14psi. I've rechecked this and its holding 14psi after a couple of weeks. Stuart turner did recommend increasing the pressure as this sometimes solves the problem but even at 18psi its still the same.
Any ideas?