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I've been riding my MTB tubeless for the past year without any issue, but for the past couple of days I've noticed that the tire deflates heavily once on a ride. I can't give you exact measurements, but I ride with 2 bars when I leave home and when I'm back home it's probably around 1 bar. It's so bad that I feel the tire 'swimming' when I corner/jump.

I've readded tubeless fluid and tightened the valve nut (I noticed some fluid leaking there), to no avail. I didn't notice any damage on the wheel or the outside of the tire.

What could be the reason that a tire becomes leaky over time? The tire is quite old (~2 years) and also not tubeless-ready (it's a Nobby Nic), but I never had issues with it for the many months I ran it tubeless. Could it be that the tire is just 'too old'?

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  • Did you clean out the old sealant before adding more? Were they the same sealant brand/type/chemistry ?
    – Criggie
    Apr 25 at 11:17
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    Did you smash the rim on something? Possibly the rim was dented, or the seam split open a little.
    – MaplePanda
    Apr 25 at 22:20
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It could be anything. Submerging the wheel in water will be useful. You might need a new valve, new rim tape or a new tyre. The air may be leaking around the spokes, around the valve, through a hole that is not properly sealed or just through the tyre material (least likely).

Be aware that a hole that is not properly sealed might be leaking air only during the ride when the tyre material is being deformed.

I do not thing you can get a definite answer without testing the real physical wheel.

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    Thanks, I figured this might be the answer. I'll try submerging the wheel (or using the detergent trick cause I'm also lazy) to see if I can identify leaky spots.
    – PejoPhylo
    Apr 25 at 7:59

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