You can probably solve that by adding a bit more brake fluid to the system. No need for a full bleed.
This shouldn't really work. Or rather, without ninja level bleed tricks I don't understand why it would do anything.
Well, if it’s shimano is pretty straight forward to add more mineral oil to the top chamber.
If the bike has been out riding I doubt there are any bubbles left in the hose, so it’s just a matter of topping up with mineral oil.
@Howard started
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You can probably solve that by adding a bit more brake fluid to the system. No need for a full bleed.