New Pressure Switch won't turn off submersible water pump.

HI
About 3 weeks ago I realised I had no water. When I checked the pressure in the shed it read at zero. I opened the pressure switch and manually activated it - it went up to 5.5 Bar (about 80 PSI) but did not cut out.   I bought a new Square D pressure switch and installed it.  But it keeps the pump running all the time - The pressure is still going up to 5.5 Bar and the pump is not cutting out.  (I turned it off after 5 minutes because I was afraid it might get damaged)
Thinking that the new pressure switch was faulty, I replaced it with another new one.  Same result.   I am now controlling the pump at the main switch as I am afraid to leave it running all the time.  This of course means that I have to manually turn it on and off when using water.  
The house is 18 years old and the pump is the same age.  The expansion tank is only 2 years old. The pressure switch is new (I have not adjusted it in any way from factory settings). 
I'd appreciate any suggestions.