Been a while. Anyway I want to temporarily drive with the vacuum hose to the modulator clamped off. It's a stock 3-speed AT. Engine is modded 280Z. I'm used to shifting manually but concerned by stuff I read. Will it hurt anything? Any tips on driving and shifting without it? Only driving 20 miles or less.

Assuming the modulator valve is bad, why not just replace it.  A new one is not expensive and easy to change.

Other than that, give it a test run around the block to see if it will let you shift thru the 3 gears manually.  Shouldn't cause any problems for your 20 mile trip if it will let you do that.

Make sure the fluid level is up to snuff since it sounds like the failed modulator was allowing fluid to be sucked out of the transmission by the vacuum line before you clamped it off. 

Did a rabbit happen to take your modulator, too?

Yes I'll try it round the block first. Maybe the rabbit or a young malnourished child assembled the two that failed.  Yes they were drinking and smoking ATF. Or possibly the bigger engine makes too much vac for the little modulators. I've got one for a 280zx to try next. My jack and jack stands are too short though. Difficult to get under the trans and it's hard to work on so close to the collector so I'm taking it where there's a lift to install the new one. Hurt my shoulder installing the last one. If my arm was a foot longer I could do it easy from the engine bay.

Happen to have a Nissan automatic pulled from a running car. Just for fun, removed the modulator and tested it with my vacuum pump - holds vacuum.    Let me know if you need it ....

Thanks. I trust old Nissan parts more than new "globally sourced" parts. My new modulator says made in USA but we'll see.

Well, it was OK but not great. There's a few seconds delay between shifting and when it actually shifts. Jerky and wouldn't shift into 3rd at all. So driving 45 in 2nd gear. And the 280zx modulator didn't fit the Jatco 3N71B. Same thread but a little wide. I think the case for the 3-speed AT's changed around 1978. The earlier ones fit the later trans but not the reverse. So ordering another. Hope it doesn't blow out this time.