adamrice 

I've got one of my road bikes set up tubeless, with Conti GP5000 TL tires. Normally, these will lose about 20 psi per week, but on two occasions, I've found one of the tires was dead flat. Most recently this weekend: Friday night, I prepped my bike for a ride on Saturday. Saturday I bailed on the ride (wind gusting up to 50 mph), but did ride Sunday. Sunday morning, I found the front tire was dead flat—it had just been sitting in my garage in the meantime.

On both occasions, I couldn't find any evidence of a puncture (no sealant spray), and the tire held air without any problem after I pumped it up. Not sure what to attribute this to.

masi61

Could you give us a bit more info about your tubeless wheel/tire combo?

Just out of left field my first guess is that your rim tape isn’t good anymore (that is if your rim requires it for tubeless). How are your valves?

shelbyfv

You might try cleaning the valves and cores, sealant goop. I think most folks pump up road tires before each ride. That would be an option even if you don't find the culprit.

cxwrench

How long since you refreshed the sealant?

Barry2 

On occasion the valve gets a booger in it and doesn’t quite seal.
maybe you blew the booger out again when you pumped it back up.

Barry

base2 

Gp5k's are usually pretty good in terms of air retention. I've never had troublesome sidewall seepage like on some popular skin walls.

That leaves the valve core, valve stem to rim, leakage between the rim & tire bead, a leak into & under the rim tape, or dried out sealant not sealing like it is supposed to. Assuming it was fine before & now it has a problem, what changed? When was the last time you refreshed your sealant?

adamrice 

So:

Wheels are "Mahi Mahi" from Just Riding Along wheels. Tape was installed by the builder. Valves are from Orange Seal, as is the sealant; valves don't show signs of being boogered-up. Wheels are about 15 months old, tires are about 8 months old. The first time this happened (months ago), I didn't check the sealant. The second time, I did, and it was pretty dry, so I added some more. But in both cases, the tire held air when I pumped it up and didn't give me any trouble after.

I do air up my tires before every ride, but in this most recent case, I aired them up Friday, bailed on Saturday, rode Sunday. I did get ready to ride on Saturday and the tires had air then, so the flat happened after Saturday morning. Weird. It seems hard to attribute that to dry sealant. Or anything else I can think of.

dsaul

Any time that happens to me, I know the sealant has dried out in that tire.

DaveSSS 

The only time I had that problem was due to dried up sealant. Just added more and all was fine. Mine never lose 20 psi per week.

2old

My guess too.

Iride01 

You normally loose 20 psi per week? Per tire or aggregate of all your bikes, and I hope that is more than one bikes worth of pressure loss.

sean.hwy

Just submerge your tire in pool of water like your bathtub and look for the air bubbles. That's we did for car tires when there was an obvious nail/screw in it.

KerryIrons

Not a tubeless user or fan, but my tubed tires lose 7-8 psi per week. I am always confounded by all the people who say to pump your tires every day. I need 2-3 pump strokes per week to keep things properly inflated (Michelin A1 airstop tubes in Conti GP 5000 tires).