It wont be the pressure switch on the pump as that would stop power getting to the varistream = no lights at all.
I had the same problem, the light flashes when there is not enough power (low batt) but also too much power. Measure the voltage on the terminals of the battery, if it's 16v or more that will be your problem. Too much power.
How is the battery charged? I have a split charge relay that kicks out too much juice. It would work before a journey but not after just flashing green light. So solution take the fuse off of the relay.
Simon