Looking at that kit I'm happy to see that it kinda resembles the KJ M4 design too. When I had a KJ I had 5 used mags and none of them had a leak of any sort :)

I'm going to add my 2 cents here. For those who done this and find that their mags still leak.


First:

Preparation

Preparation is the key when doing a job like this.

Degreasing every parts that are going to be in contact with the gasket maker thoroughly will do wonders and insure a tight none leaky seal.

Second:

Less is more

Too much gasket maker will reduce the mags capacity in gas as this surplus will take up valuable real estate in the reservoir.

Third:

DO NOT overtighten the valves

If the valves are tight and it's still leaking, do not overtighten them, you'll make thing worse by crushing or even cutting the o-ring. I use o-ring grease that is found in pool supplies shop. They are rubber safe and work Wonders on leaky orings.

Hope this helps