Shimano GRX RX 815 Series Dealer's Manual page 147

MAINTENANCE
SHIMANO genuine mineral oil replacement
5.
Install a bag and tube on the bleed nipple.
(1) Place a 7 mm socket wrench in the position shown in the figure.
(2) Connect the bag and attached tube to the bleed nipple.
6.
Loosen the bleed nipple.
The oil will begin to drain. Operating the brake lever while the oil drains will allow the oil
to drain more easily.
Bleed nipple
Bleed nipple
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