Edgun Eazy Fill - Too much pressure?

When I press the handle on the eazy fill, there is a 'pop' and then a big jolt of pressure shooting to my rifles. Then it eases off and I can somewhat control the flow.
Although not nearly as smooth as when I directly connect and control the fill with the main tank valve. Is this jolt harmful to my rifles. I would think it can't be good.
When using the eazy fill, I open the tank valve all the way. Should I not? Kind of defeats the purpose if I have to constantly adjust the fill with the main tank valve.
When I'm done using the eazy fill, I close the main tank valve and leave the line pressurized. However, yesterday I walked in to find that the rubber plug on my main tank gauge had popped off and it leaked out glycerin all over the place. Did leaving the line pressurized cause that plug to pop out?
If I can't leave the line pressurized, then what is the point of this eazy fill contraption?
Am I doing something wrong?
 
The flow rate of the eazyfill is adjustable. Brian has a video on how to do so.
as far as the other issue. Sounds like the valve or tank is low quality or old. I’ve left my tank wide open and the there’s to my rifles for hours with zero issues.

Brand new AirMarksman 9L tank. This is the plug that pops out if the line is left under pressure:

plug.jpg