About 2 weeks ago I replaced the stock tire on my Aggressor Pro with a Kenda Kwick Drumlin. I took it for a ten mile ride, with no problems. A few days later it had gone down from 60 psi to 50; I topped it off and went for a twenty mile ride - again no issues. Today the tire is totally flat - even though it's just been sitting in my garage since the twenty miler. I thought maybe it was another slow leak, but no - any air I put in leaks right out near the valve. What could cause it to work fine on those rides, then develop a major leak just sitting in the garage?

You obviously got a puncture on your last ride.

Couple of ideas:
You had a slow leak and now have a different faster leak. Take the tube out and feel for leaks with your cheek or with water.

The valve stem was slightly loose and is now much looser - tighten it.

You had a small crack where the stem joins the tube and the crack worked itself larger because you rode the tire underinflated because of the slow leak during the ride. Again, see if you can feel or see the air leaking out of that area.

Regardless, either assess and fix the leak with standard measures, or replace the tube. It's just a leak, figure it out and fix it.

It doesn't seem like a puncture...if I try to inflate it I can hear air leaking out of the valve. Also, I made it back from the last ride okay...either it wasn't leaking at that time or maybe had a slow leak that I didn't notice...the fast leak only developed over the past week while it was just sitting in the garage.

Did you investigate this "leaking out of the valve" thing?

If the air is truly coming out of the valve, a likely suspect is the valve core:

If the valve core is loose, try tightening it.
If the valve core is replaceable, try replacing it.
If none of the above, then send the tube to your nearest landfill.

Did you fill the tire with CO2 by any chance?

Time to take out the tube and find the leak with a bucket or sinkful of water. None of our guessing, however close, will fix it.

+1

/thread

I had a tube that would hold air for weeks at a time and then go flat on my 6 mile commute to work in spite of the fact that I always topped of the air pressure every morning before leaving the house. Took the tube out filled it with air submerged it in water and could not find a leak. Put the tube back in and wouldn't you know it a few months later it went flat again. The only possible explanation I can think of is something had to be mucking around with the valve stem, that happened 3 times, I replace the tube. But we are talking 70psi to flat inside of a 30 minute ride I still want to know what that was all about. It is just not possible but it happened.

My first guess is a loose valve core.

Next would be a breach by where the valve stem meets the rest of the inner tube. I've had those and even when submerged in water the bubbles didn't appear until pushed the stem off-center.

Next guess would be a thorn that has worked its way through the tire and is ever so lightly impinging on the innertube. When sitting still its a slow leak, when riding it leaks much more. Take off the tire and feel around the inside of it for a small, sharp point.

These have all happened to me over the years.

It sounds like one possibility is you may have tightened the valve nut too tight, which can pull on the tube and slowly tear it. But as mentioned above, pulling the tube out and inspecting it will lead you to an answer much more quickly.

Presta or Schrader valve? At least one reply here only applies to presta valves and that is tightening the nut too much.
My experience is that you would break off the top of the valve if you tightened the closure nut too much and you would see that right away. I've never lost a valve tightening the nut that may be there to fix the position of the valve in the rim hole.
You might have pinched to tube between the rim and tire when you mounted the new tire. That sometimes causes the tire to lift off and you shred the tube at that point. I agree the best way is to remove the tube and inflate it outside of the tire. I have a narrow 700C tire on my trike. It only requires topping off about every 10 days to avoid pinch flats. If it goes down any sooner than that I know I have a puncture.

Correct! I wasn't expecting someone to remotely diagnose where the leak is; I was just wondering if anyone might know why it got me through thirty miles without a problem, then developed a major leak while sitting in the garage!

Wrong. Specialized is just one brand that has lockrings on Schrader valve tubes so there's millions of those out there.

I didn't know that valve cores were a thing, so I checked that out - but it turns out the tube didn't have a removable core. So I put in a new tube and all is good. It turns out there was a very small puncture about an inch from the valve (not where the valve joins the tube, as I originally thought). VegasTriker may have hit the nail on the head...I'm thinking maybe I might have pinched that area when I mounted the new tire, and for some odd reason it took a week before it became bad enough to do any serious leaking. Thanks to all for the advice!

No...I've never tried a CO2 inflator but I'm thinking of getting one - I can only get about 40 psi with my flimsy little portable pump.

Mounting the bead at the valve stem last, is a good way to get the tube pinched under the bead, and make your tube explode. Then you can come back here and ask "why did my tire blow up?"

The reason I asked was because CO2 diffuses through butyl rubber, so your tire gradually goes flat with no hole in the tube or defect in the valve.

Hmmm...so maybe the way to go is CO2 for a roadside fix, then refill with air once back home with a decent floor pump.

Air as well as CO2 will diffuse through tubes over time. CO2 just does it a tad faster. So don't expect your tire to be inflated to the proper level if you leave it for over a week.

Though larger volume tires with their lower riding pressures won't lose the same amount of pressure as my skinnier 25 mm road tires. I usually have to fill them every other ride. Or roughly every three to four days to have them up to their pressure for riding.

CO2 diffuses through butyl tubes a lot faster than nitrogen or oxygen.