Shimano GRX RX 815 Series Dealer's Manual page 156

MAINTENANCE
SHIMANO genuine mineral oil replacement
11.
Once no more air bubbles come out from the bleed nipple, temporarily
tighten the bleed nipple.
12.
With the lever depressed, loosen and tighten the bleed nipple in rapid
succession.
Loosen and tighten for approximately 0.5 seconds each time to release any air bubbles
inside the caliper.
Bleed nipple
Bleed nipple
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