My GX CS4 filling guns, but will not fill my scuba tank.

As the title states, my newish GX CS4 is not filling my scuba tank. The tank is 6.8l, 300bar. So I have been using this compressor for two months and it fills quick and stays cool. Love this thing. But with my new Panthera 700 I thought I would up my air game and get a tank. While filling however it seems to build just enough pressure to the get to the 1 line on the reading but then drops back down and this fluctuation keeps happening every 3 minutes or so. I double checked and it’s still filling the gun no problem. Lastly, when I first attempted to fill it when I got the tank I ran the line from the compressor to the output on the regulator by accident and realized a bit later and switched to the correct input. I’m wondering if putting the air through the output would’ve just trashed my regulator or something? Outside of that though if there’s any suggestions that would be great, id like to get to the bottom of this.
 
A picture of the Is valve on your bottle would be worth a thousand words
Okay when I get home from work later I’ll get a photo. These are from the product photos on Amazon

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If it's still filling your guns just fine, then I would suspect the compressor is fine and something is amiss with the tank valve. It is very possible the regulator was damaged by the negative pressure from trying to fill the tank through that port and now it's leaking from the regulator and not building pressure. I would check for leaks on the valve and then tear into the regulator and see if an 0-ring may have blown due to the pressure coming from the wrong direction.
 
Thanks man! Appreciate the help and I’ll disassemble it this afternoon and see what’s up. Also what’s your suggested method for checking leaks?
If it's still filling your guns just fine, then I would suspect the compressor is fine and something is amiss with the tank valve. It is very possible the regulator was damaged by the negative pressure from trying to fill the tank through that port and now it's leaking from the regulator and not building pressure. I would check for leaks on the valve and then tear into the regulator and see if an 0-ring may have blown due to the pressure coming from the wrong direction.
 
Thanks man! Appreciate the help and I’ll disassemble it this afternoon and see what’s up. Also what’s your suggested method for checking leaks?
I typically mix some dish soap with water and spray it on the various parts and watch for bubbles. There are specific leak decking fluids available on Amazon, like Better Bubble, that are supposed to work better, but soapy water should work in a pinch, if you don't have the other. If the tank is not holding any air, you may have to connect it to the compressor and have the compressor running to build a positive pressure in order to see where the bubbles are forming.

 
Looks like the same valve that came on my 1.6 l Hatsan bottle. Take the line from the compressor and plug it into what they have labeled as 8 mm male quick plug with filter. Make sure your bleed valve is closed on the compressor and on the bottle. Turn the compressor on and it should fill the bottle. it would take a long time on your larger bottle
 
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If the check valve is stuck shut then the compressor would quickly build to the shutdown pressure as it's only pressurizing the hose. It sounds like either a leak in the fill valve or as @jvrobert suggests, are you filling with the tank valve left open? A CS4 is only filling at 11 liters per minute which would not make much sound to indicate it is leaking out as fast as the compressor is pumping. Is the bleed valve closed when you fill your tank?
 
If the check valve is stuck shut then the compressor would quickly build to the shutdown pressure as it's only pressurizing the hose. It sounds like either a leak in the fill valve or as @jvrobert suggests, are you filling with the tank valve left open? A CS4 is only filling at 11 liters per minute which would not make much sound to indicate it is leaking out as fast as the compressor is pumping. Is the bleed valve closed when you fill your tank?