Tubeless valve leaking, what exactly is wrong & what do I need to do to fix it?
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.