Cherne Clean Seal OK in Brass Test Tee?

I'm having a heck of a time sealing up my copper DWV system to perform a leak check.

I got my hands on a Cherne Clean-Seal screw-in inflatable plug, and installed it in a 2" brass test tee. I've filled it to the recommended 35 PSI, then to 40, and it's not holding water very well. I've poured 15 gallons in an upstairs tub drain, and only been able to drain out 2-3 gallons out of a capped sink drain directly above my test tee on the ground floor.

The Clean Seal is rated for 30 feet of head, and my installation is something shy of 20. The test tee is located just underneath a wye that joins a 2" drain and a 1½" vent, so both are being filled when I'm dumping water down the tub drain. But I'm expecting this shouldn't result in the problem I'm having.

I noticed the instructions state the Clean Seal is intended for cast iron and plastic -- brass isn't mentioned -- so might this be my problem? How much more can I safely inflate this in an attempt to create a seal?

BTW, I've also got a conventional inflatable plug, but I'm reluctant to use it due to the terrible mess I make every time I release pressure to drain the line.
http://www.cherneind.com/

It SHOULD work, but it depends on the smoothness of the tee around the opening, and whether the tap is raised from the tee so the plug cannot reach the back side effectively.

Doh!

Thanks. You're right, it worked fine -- and was working fine all along.

The water was backfilling up the vent, as I expected it to, but then hit a vent branch line in the ceiling and was draining somewhere else.