My fix was to use some medium viscosity super-glue to build a fillet at the joint between the plastic and brass. The super glue appeared to creep down into the joint and seal it. After sealing in this way the TVSV would hold a vacuum on the lowest port.
Drivability: The TVSV in the car was removed and testing found it to be OK at low (<45 F) and medium (65-122 F) temps, but not at the high temp. Apparently the internal shuttle was not moving far enough (see picture). I had another TVSV that was cracked so I pulled it apart to see what it looks like.
The pin in the base apparently pushes out more at higher temperatures pushing the spool-shaped shuttle further up in the plastic cylinder. The shuttle is rubber and seals against the inside of the plastic cylinder. As the shuttle moves it opens or closes ports on the cylinder.
I found that the defective TVSV would work correctly if I heated the base in the flame on the stove. I gave it several cycles of heating an cooling to see if the shuttle could be induced to move further so it would work correctly at lower temperatures but it never did work at the correct temperature.
When I replaced the TVSV with one that worked correctly I found that the car had a much smoother off-throttle to on-throttle transition at small throttle openings. Prior to replacement the car would tend to violently buck or surge during low speed, low rpm throttle transitions. This occurs when almost coasting at 25-30mph in 3rd or 4th gear and low rpm. This has happened in other T4wds I've driven so it seems pretty common.
