"How does that work? I thought the non-bladder tank had zero air pressure when the pump is shut-down and the water is bled."
A non-bladder tank can be precharged with air to 2 psi less than the ON setting of the pressure switch. You should make the water connection to the tank at the drain level, just as is done with a bladder tank. Empty the tank to that level; close the discharge valve; and pressurize to the desired pressure. Then run the pump to shutoff pressure before you try to use any water.
Maintaining the tank is even easier. You need a means of determining the water level; either a sight glass tube, or by observation of the sweating tank, or by tapping on the tank. The level of water in a tank corresponds to an air pressure for a correctly charged tank.
For a 30/50 tank, using the practice of 2 psi less than the switch setting when empty, the pressure when empty of water will be 28 psig = 43 psia (at sea level). That is also the point where the tank is full of air (100% air).
psig = psi gauge (what you meaure)
psia = psi absolute = psig + atmospheric pressure
Now when the tank shuts off at 50 psig (65 psia), the volume of air in the tank will be 43 psia/65 psia = 0.66 = 66%.
So when the pump shuts off at 50 psig, the water should be at the 34% level.
To maintain the air charge when there is too much water, run the water down to the 34% level and add air to 50 psig.
So here are the water levels for a 30/50 tank with 28 psig at empty:
0% 28 psig
5% 30 psig
10% 33 psig
15% 36 psig
20% 39 psig
25% 42 psig
30% 46 psig
35% 51 psig
Now you can observe the level at any time and adjust the air pressure to get the correct air in the tank. If it is above 35%, you should let some water out of the tank to the 35% level and pressurize to 51 psig.