Notice in this video of a real P-51 he turns on the fuel pump at the 5:13 mark.
Notice the switch is Off in the up position and On in the down position, You can see the fuel pressure come up when the switch is moved to the down position (On)
Then notice after takeoff in this video, he turns off the fuel pump at the 1:50 mark and verifies the fuel pressure is good. The fuel pump is then turned back on at the 6:14 mark before landing.