When landing valve is open do the network pressure reduces even the main pump is running?

The main pump will run depending on pressure setting and sequence of operation set ,,regardless of which device is opened to release pressure. 

REPORT PUMP OPERATOR AND SUDDENLY OFF THE SYSTEM AND REDUCE PRESSURE AND  SHOULD CONNECT  A PRESSURE RELIEF VALVE. 

PRV (pressure regulating valve) Will regulate the pressure at stable value

When the landing valve is open, the network pressure reduces which initiates the pump to run to maintain the network pressure.