Espresso machine pumps should be connected to a water supply with a steady incoming pressure between 1.5 and 4 bars. The pressure in the water supply is called the ‘line pressure’. If the line pressure is too high, then a pressure regulating valve (PRV) should be fitted to reduce the incoming pressure to meet the manufacturer’s specifications.
The espresso machine pump adds a fixed amount to the line pressure that’s already there. This means that if the line pressure changes, the brew pressure will change, too. If your line pressure tends to fluctuate, then a pressure-regulating valve (PRV) can also help. The PRV should be set to a level at or below the lowest line pressure you get.