Observations:

The vacuum peaks out at 20 Hg, and pressing on the brake pedal drops 5 Hg, then releasing brake pedal drops another 5 Hg. After the pedal is released, the vacuum builds back up (even with foot off accelerator). However, comparing my 300TD to a 300SD that has great brakes, the difference is that the problematic car won't build vacuum (though it stays steady) until I let go of the brakes. On the better car, the vacuum builds even while the brake pedal is being held down.

When I have the car off, the pedal goes about half way before it sticks, and when I turn the car on, the vacuum drops 5 Hg (as normal with brakes being held) and then the pedal slowly moves closer to the floor. The vacuum doesn't drop any more. I'm not sure if the pedal not being able to hold resistance means an issue with the booster or the master cylinder.

On my other (better) car, the brakes stay firm even as I hold the pedal down.