ARB sucking up fluid

What would cause the diff fluid to be sucked up through the ARBair hose?
When I deactivate the ARBthe puge of air also has diff fluid in it and a lot. It is acting like a straw.
I do not use it that often and I was just testing it to make sure it still worked. I am also getting ready to do fluid changes and was wondering what I should look for, if anything, in the diff.
Dana 30, front and older style ARB.
Thank you

P.S. Can I flush the line myself and with what?

I've used carb cleaner to flush the line. I take the axle end off first though and just leave what's left alone.

Couple of causes and has it done it in the past:

1. Vent line is plugged causing pressure to build in the diff.
2. Orings or piston leaking.

Only seen it as a problem for the rear where the vent line is in the tube, but I'll add it anyway.
3. When the diff was installed there wasn't a relief around the race to allow the diff fluid to easily drain back. The tube gets full and starts bubbling up into the vent line eventually causing it to plug. Relates to #1 above.