ok,
the brass screw on the end just controls your output pressure, the reason it is reading low-p is because the pressure in the tank is now below 200 psi ( the lowest the sensors will read, might be 250?) so twiddeling the little brass screw aint gonna empty it.
what i do is slowly unscrew the allan screw on the right hand side with the little silver plate under it (its an automatic safety release valve that leaks when your output pressure gets above 900 psi) un screw it slowly a half turn and you will hear the remaining compressed air seeping out.
now, mind that seeing as you have turned the little brass screw on the end, you have altered your output psi and will have to adjust it when you next fill your A.I.R , keep it around 650 psi (output) an you should be shooting sweet!
phew!