Shimano GRX RX 815 Series Dealer's Manual page 102

INSTALLATION/REMOVAL
Installing the brake hose
2.
Add a cut mark.
Mark the hose at a position 21 mm from the check mark towards the end of the hose.
3.
Prepare the SHIMANO original tool TL-BH62.
Disassemble the tool as shown in the figure.
[1]
[3]
21 mm
Cut mark
Check mark
[2]
[4]
Main body
Press block
102
Hose cutter