Grease gun and oil separation

[dmjlambert]

This is the grease gun I bought, it's a Lumax.



I bought a cartridge of Valvoline SynPower Synthetic Automotive Grease and put it in the gun.
Since the last time I used it, it has leaked quite a lot of oil. The oil is separating out of the grease at a pretty much crazy rate.
I have since read online that I should store the gun such that the grease is not pressurized.

Before I go out in the garage and make a crazy mess and have a grease gun accident, I would like to learn from you guys how to do this properly. I have read online in a variety of sites and watched videos but it is not clear to me. I've also seen conflicting information about storing the grease vertically or horizontally.

If I pull out the t-handle as far as it will go, which seems to be about half way, does that release the pressure such that the top of the gun can be unscrewed and the cartridge removed? I have noticed the t-handle will not push back in unless I press the lever while pushing the t-handle, so I suppose that lever holds the t-handle's rod in place.

There is obviously a spring in there that is under tension when there is grease in the gun. Should I remove the grease cartridge and remove tension on that spring when I'm not going to be using the grease gun for a long time, such as several months or a year?

This half-used grease cartridge has had so much oil separation that I'm thinking I would not like to use it in my truck any more, because it is thick. How should the grease cartridge be disposed of? Do municipal household hazardous waste collection sites typically accept them?