Shimano GRX RX 815 Series Dealer's Manual page 157

MAINTENANCE
SHIMANO genuine mineral oil replacement
13.
Repeat step 12 two to three times, and then tighten the bleed nipple.
14.
Set the lever in the position where the bracket is at 45°, as shown in
the figure.
Perform adjustment by changing the angle of the handle, etc.
Bleed nipple
45°
157