Psi does not go above 20

Check drain as it might not be closed all the way, you may have to set the regulator and there could be a bad switch diaphragm.....

Even with 3 gal capacity it should stop at 120psi so I'm guessing a good 3mins to recharge the vessel. When pressure hits 90psi that's when the motor kicks in to refill. What you have to worry about is not stopping and charging the vessel to dangerous levels and not activating the pressure release valve. Let it run a good 3mins then manually shut it of and unplug it from the electric source. Using a bottle sprayer with a mixture of water and detergent, spray around the cylinder head, the plumbing and the bottom drain valve. Watch if bubbles come out in those areas and show you where it's leaking. Remember liquid conducts electricity easily as well as any copper wire, so wait till it's dry so it wouldn't short out or electrify you. Never use a compressor without an analog pressure readout and pressure regulator. A water separator would be a good idea to add later on.