The only thing different between these 2 is the threaded fittings. The gasket is what seals these units so I cant see what could possibly be the problem with what you were doing unless youi had 1 with a bad O ring or maybe you were putting it under a much hoigher vacuum? We only go up to 22" of vacuum with these pumps and I know mine holds that no problem. Actually we arent even puting that much vacuum on one as we are racking the wine not degassing it so probably no higher then say about 12" of vacuum can really be achieved during this.
I didn't have a bad o-ring. I actually tested two of the filters, both brand new, both with perfectly sound and new o-rings.
The gasket is a floating design. It sits in a U-shaped channel at the top of the sump and there is no possible way it can be compressed by contact between the sump and the head. It is protected from compression by the channel it sits in. It seals by internal water pressure. I suspect that an internal vacuum unseats it from the channel wall.
I don't have a vacuum guage on my Harbor Freight vacuum pump, so I can't say how many inches I was pulling. However, I used an air valve bypass to decrease apparent vacuum (based on visual and auditory evidence), and it made no difference. In every case, the filter sucked air, with or without a filter installed in the housing.
Again, I'm not an engineer. But I tried for several days and in many ways and I could never eliminate the air leak with that Pentek housing. I removed the red button on top and sealed the hole. I tried double o-rings. I added homemade gaskets of varying thickness made of rubber or vinyl between the sump and the head. I did everything I could think of but permanently glue the head and sump together, which wouldn't be practical anyway.
It sucked air.
And both of the filters I bought are now in use under my sink, one with a sediment filter, the other with a charcoal filter, feeding a tap on my sink and my espresso machine. Neither one leaks a drop of water.