POTTERTON PUMA COMBI?

WAS HAVING LOW PRESSURE WARNING LIGHT ON, SO HAVE REPLACED WATER PRESSURE SWITCH, NOW GOT HOT WATER, BUT HEATING ONLY LASTS FOR APROX 1 MIN THEN THE BOILER SHUTS DOWN AND LOW PRESSURE LIGHT COMES BACK ON. I HAVE DRAINED THE BOILER THEN TOOK THE SWITCH THEN PUT IT ALL BACK TOGETHER AGAIN, JUST IN CASE I DID SOMETHING WRONG THE FIRST TIME,
REFILLED THE SYSTEM THEN FIRED THE BOILER BACK UP AND THE SAME HAPPEND... HOT WATER OK HEATING APROX 1 MIN...
SO HELP PLEASE...