Marui valves are supposed to vent gas (not liquid)
in this image, you can see a small hole near the threads of the fill valve. This to to allow the gaseous portion of the propellant to escape out the side of the fill valve while the long tube allows the liquid portion to dispense out the bottom. When filling, you should get a small spray of gas coming from the fill valve. When the mag is full, this spray will turn liquid, letting you know that the mag is full.
As far as I know this feature is only present on TM gas mags.