I have an Alkin W-31 Mariner vertical compressor and am trying to fill a SCBA tank w/ a Saber Tactical Valve. Following the Alkin instructions exactly. Start compressor w/ fill line valve closed and tank valve closed. Open fill line valve, wait until fill line gage reaches fill pressure (300 bar), then open tank valve. As soon as I open the tank valve the tank gage reads the same pressure as the fill line gage 300+/- Bar. Is that correct? I thought the tank pressure should read lower and then slowly come up.