These aren't catastrophic blowouts. You don't hear air escaping. It isn't like everything is fine one moment and the next the tire is flat. Eventually, the bike just starts to feel sluggish and handle poorly. These are slow leak flats. That means teeny tiny holes.
Ordinarily, after replacing two brand new tubes and having them go flat, the simplest thing to do next is replace the tire also. But, I ride Continental Grand Prix 4 Season 25 mm tires. They are great tires. They often last more than 3,000 miles without a single flat! And, these miles were on roads I've ridden over many times before with other brands of tires and fairly routinely wound up getting flats.
So, I like these tires a lot. I've had good success with Specialized Armadillo tires too. But, the Continental Grand Prixs handle quite a bit better too. They also cost more than my car tires; $50-65. So, just replacing one because I can't find the cause of a flat isn't a great solution. At any rate, unless you happen to also carry an extra tire on your rides (I often do), replacing the tire is only an option if you happen to flat within walking distance of bike shop.
Finding the Cause of the Slow Leak Flat while on the Side of the Road
Even before you remove the tire from the rim, take a good visual inspection of the tire and then also run your fingers over it, both the main tread and sidewalls taking care to avoid cutting your fingers with any sharp debris that may be protruding from the tire. Sometimes you will find a tiny bit of metal but that is obviously not the cause of the flat because it isn't poking all the way through the tire. Sometimes, you will find nothing on the exterior surface of the tire because its actually embedded itself within in the tread.
If you're lucky, you've only spent a few minutes looking for the cause of the flat and can now confidently replace your tube knowing you've corrected the problem. However, if you didn't find the cause from inspecting the exterior of the tire, in all likelihood the problem is small bit of metal on the inner surface of the road side of the tire.
Next, start by removing one bead of the tire from the rim all the way around. If you are in quiet conditions where you could potentially hear air escaping from a small leak, it might be possible to inflate it a bit and then listen for air escaping. Alternatively, you can to remove the tire completely from the rim but keeping the tube and tire fixed in their relative positions and try inflating and listening for the leak that way. If you can narrow in on the leak by listening for escaping air, this helps to find the section of the tire where the problem is and you can mark it with some scratches on the tire surface.
Next, you can remove the tube from the tire but do so in such a way that you can remember (approximately) their relative positions. Inflate the tube by itself and then hold it near your ear listening for and feeling for escaping air. If you find the hole on the tube and if you remember the tube's approximate relative position to the tire, you can then identify the section of the tire where the cause of the flat must be.
Otherwise, you will have to inspect the entire inner surface of the tire as well. Its best to feel with your fingers. Curl your fingers inside the tire and slowly slide them all along the inner surface. But, be careful! If there is something sharp poking through, you can cut your fingers. Use your fingers to feel all along both the walls of the tire as well as the road-facing part. Finally, take one of the tire beads, pinch it between thumb and forefinger and run your fingers around the entire circumference of the tire. Then, do the same for the other bead.
At this point, if you still don't have a good candidate for what is causing the flat, make another pass over the tire inner surface. This time, however, do it in sections of a few inches each. Grab a section, turn it inside out (as much as possible) and then flex the tread back and forth. Eyeball the inner and outer surfaces as you flex it and then after eyeballing, again run your fingers over the surfaces. Small bits of metal will work their way out and reveal themselves when the tire is flexed like this.
The last thing to check is the rim. Again, use your fingers and slide them all along the rim surfaces that contact the tire. Also, visually inspect all the spoke holes to make sure they are covered with rim tape and nothing sharp is possibly contacting the tube surface. If there is a problem with a spoke poking through the rim tape, you can temporarily remedy that with a few layers of paper (even paper money) or a tube patch.
If you still can't find the cause and if the leak is slow enough, then I hope you have a pump and not just CO2 cartridges. That way, you can still limp home by stopping every so often and re-inflating your tire.
Finding the cause while at Home
When you are back at home, you have one additional tool you don't usually have at the side of the road; a sink full of water. If you've tried everything described above and still can't find the cause of the flat, you can immerse sections of the whole wheel, rim, tire and tube, together in water and watch for bubbles.
However, its best to have one bead of the tire off the rim and the tube just partially inflated. If you have the tire wholly on the rim and the tube fully inflated, escaping air often travels along the rim to the valve stem hole where it escapes to form bubbles leaving the false impression that the cause of the flat is at the valve stem. So, remove one bead of the tire from the rim and partially inflate the tube. Then immerse sections of the wheel in water and watch for bubbles. Wherever the bubbles are, you are near the section where the cause of the flat is. Mark it with a Sharpie, then remove the tire and start looking at it with a fine tooth comb. You'll find it.
However, its best to have one bead of the tire off the rim and the tube just partially inflated. If you have the tire wholly on the rim and the tube fully inflated, escaping air often travels along the rim to the valve stem hole where it escapes to form bubbles leaving the false impression that the cause of the flat is at the valve stem. So, remove one bead of the tire from the rim and partially inflate the tube. Then immerse sections of the wheel in water and watch for bubbles. Wherever the bubbles are, you are near the section where the cause of the flat is. Mark it with a Sharpie, then remove the tire and start looking at it with a fine tooth comb. You'll find it.
On a few of occasions in the last 30 years of riding, I've had such hard to find flats. When this has happened, ultimately the problem was a tiny bit of metal wire embedded in the tire tread such as you see at the end of the pliers in the picture below.
It'd be a shame to toss out an otherwise perfectly fine $50 tire due to such a small bit of metal!
