Hi. Just a word to say that the problem of this thread has been resolved with the suggestion of Bob NH. I could be useful for someone else.

http://www.terrylove.com/forums/showthread.php?t=14767

The little plastic tube that goes from the pump to the pressure switch was completely plugged with a mix of sand and dirt. Even the hole where the fitting is screwed to the pump was plugged.

I've changed the switch before but never think that this hose could be plugged.

Thanks again.