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My front tyre has started to leak air at the top of the presta valve.

It's very noticeable when the valve is unscrewed or even not really tightly screwed in: you can feel a breeze coming out of the top.

When I screw the valve cap around past max tightness it slips & lets out a hissing noise.

Even with the valve tight enough that you can't feel or hear anything, the tyre runs dead flat after about 2hrs.

I'm pretty new to tubeless, and I've never had a valve act like this before.

The tyre is a Gavia AC 2. What do I need to do to fix it?

edit: looks like I might be loosing sealant, and need a truing - the rotor is rubbing the calipers at one point :( enter image description here

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When I screw the valve cap around past max tightness it slips & lets out a hissing noise.

The most likely thing happening here is that your valve has a removable core, and you have slackened it off. You can get special tools for the job, but usually the valve core has a small flat area that you can grip carefully with some pliers to tighten it again.

enter image description here

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  • Thanks! I've tightened it and for a minute I could hear a bubbling noise, which went away when I tightened further. Is it safe to tighten until it won't go any further or is there a risk of damage at some point? Do you know if this is something that could be defective, or is this usual behaviour and I should regularly tighten the cores as part of routine maintenance? – Scottmeup Nov 28 '19 at 12:37
  • It's probably not defective, it's fairly common. As to how tight is too tight - i'm not really sure to be honest, I just tighten until it felt snug, but without applying much physical strength. I've never felt the need to check them as part of routine maintenance. – Andy P Nov 28 '19 at 12:48
  • There's a special tool for that purpose. It's preferable to pliers because there is still a risk of damaging the insert. BTW All tubeless valves have a removable core to fill latex milk into the tyre. – Carel Nov 28 '19 at 18:02
  • Over-tightening will strip the metal where you apply the tool. That happens to any bolt or screw, the smaller it is, the easier it is to damage the metal. – MarcClintDion Nov 29 '19 at 2:44

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