Shimano GRX RX 815 Series Dealer's Manual page 159

MAINTENANCE
SHIMANO genuine mineral oil replacement
17.
Operate the lever.
Air bubbles in the system rise up through the port into the oil funnel. Slowly repeat until
no more air bubbles appear.
18.
Check that the lever has become stiff.
NOTICE
• If the lever has not become stiff, repeat the procedures from Step 10.
Loose Slightly stiff
159
Stiff