It sounds like your pump is set for 20-40 psi cut-on/cut-off. If you're reading the presssure tank after it runs until cut-off, all that you're reading is the cut-off pressure (40 psi).
The proper way to check the tank is to turn off the pump, drain the water pressure down, and then check it. It should read two psi below (18 psi) the cut-on pressure.
If it's too low, air it up with a bicycle pump or portable air tank or compressor. If it won't hold that pressure, you probably have a shot bladder, and the pressure tank needs to be replaced.