Bladder problems???

I have a water pressure tank that has been cycling on and off. When I checked the pressure it read at about 13 psi so I added air up to about 28 - the switch is set at 30-50. It retained the air pressure for a few hours and worked like it should and then it dropped down to about 13 psi agian.
Water does not come out of the schrader valve, but I think I have a torn or leaky bladder. Any thoughts?
If it is, any recommendations on replacing the bladder or replacing the tank? I am guessing that it is about 15 years old.
Thanks- TVS

If the bladder tank does not maintain the air pressure, throw the tank away and put in a new one. Your pump will thank you for it.

Like CC said, if the tank doesn't maintain air charge, you need a new one. Replace with a tank at least the same size or larger, or you can switch to a CSV and small tank like the picture to the left.