Tubeless valve leaking, what exactly is wrong & what do I need to do to fix it? 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 &amp; 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 :(
When I screw the valve cap around past max tightness it slips &amp; 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.