Well that sucks...
I rode my Serotta out Monday for the first time since last fall. Tuesday was brutal, so I just puttered around town on my Dawes. Yesterday I rode the Dawes again. The ole thumb check on the rear tire suggested there was still plenty of air in the Panaracer Race C Evo3 tubulars, so off I went. After a few miles the front tire started to feel real mushy, so I stopped and added some air. That got me up to Highland Park, where I decided to ride down Ravine Drive to the lake. On the way back up, the front tire nearly went flat. I walked up the steepest part, then put in some more air. That got me a couple more miles, where I decided to just put on my spare. I opened the valve to let out the last of the air only to find my hands covered in sealant. Hmmm... Why didn't the sealant work? More important, why didn't any sealant leak out through the treat or sidewall? This didn't sound good.
A few minutes ago I decided to investigate. I pumped up the tire and it went flat almost immediately. Then I looked at the base tape and saw little droplets of sealant puddled along the center. Apparently, the sealant had oozed out of the stitching and through the base tape (now that it wasn't glued to the wheels. But wait, there's more...
Thumb on the front (spare) tire, feels good, about a day's worth of air leakage. But the rear tire is completely flat. I pumped it up. It seems to be holding air for now. There doesn't appear to be any sealant in the tube.
So, I'm not 100% positive, but I suspect there's an issue with the valve-stem-to-tube interface. These tires have relatively few miles on them. I think I put them on the Serotta new when I was building it up. Strava tells me I have just over 720 miles on that bike. That would be the max on the tires. I bought them from Bike Recyclery in Portand in Feb '21. Given the nature of their business, I'm not sure approaching them would be fruitful. I guess I'll just chalk it up to experience. I'm unlikely to ever buy this particular tubular again.