UPDATE: After many tests and isolations of systems-

The new one holds vacuum perfectly, and does not leak-down noticeably.

I didn't know the cap press-snapped on, there are no instructions with these and somehow I overlooked that when testing. So I will be replacing the modulator soon!

Also, I capped the line to the Modulator, and everything vacuum now works in the car- woohoo!

The modulator is very hard to get to, I have to disconnect the front of the driveshaft at the trans, and take the trans mounts off to let the trans sag enough to get up in there. Any tips appreciated-