Shimano GRX RX 815 Series Dealer's Manual page 166

MAINTENANCE
Replacing the brake hose
2.
After checking that the olive is positioned as shown in the figure,
apply grease as shown in the figure.
Brake hose
Connecting bolt with flange
3.
Install the brake hose in the lever.
Secure the lever to the handlebar or in a vise and insert the brake hose straight.
Insert the hose until the check marks added beforehand on the surface of the hose are
covered.
Grease
Check mark
166
2 mm
Olive
Connector insert