Fred your kinda right on the tire sizes, first is tire section width, the second is aspect ratio. Those are the proper terms.
On an inner tube the sizes are just width.
And as I have mentioned before there are 25.4 mm to the inch. So to convert from mm to inch divide the mm by 25.4. in this case 80 = 3.14 Or in tire size a 3.10, 100 = 3.93 or 4.00 So a 80/100 21 fits a tire from 3.10 to 4.00.
If you can't find a leak in the tube itself it may be a bad valve stem core. You can get them most anywhere, even Wal Mart. Look where they sell air compressors and tire pressure gauges, They often have car valve stems too.
A few bucks for a pack.
I always save the ones I take out old tubes and the valve stems I replace when I replace them on car tires I replace. I mount all my own tires. Bike car, van, truck, trailers, boat and flat bed for hauling bikes and such.
Leo