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So Cody and I got the tachometer working tonight. Big thanks to @cgsheen1 Chuck. His explanation for current flow to the tach and back made it all make sense. I took the tip of my thumb off with a hammer tonight in the shop, so I didn't get the last piece of leather in. Opposable thumbs are really useful, as it turns out.

The reaction disk won't just fall out all by it's own. If this problem occurred all of a sudden, it's not the disk. A torn booster diaphragm will cause a hard pedal, and so will a malfunctioning poppet valve inside the booster. Either way, you're opening or replacing the booster.

I agree with Patcon. I'd cap off everything you don't really need right now, including the brake booster and the transmission. And yes, I think a vacuum gauge could be helpful. You should be in the 17-20 inches of vacuum at idle. If you're at 10, there's a leak somewhere. I also think pics could be helpful.

Oh, and I'd be hard pressed to believe it's a malfunctioning check valve. If you're off the gas pedal when you're on the brakes (as you most likely are), the check valve won't really matter because the manifold vacuum is high (because you're off the gas pedal). Does the idle go up a little bit when you press the brakes sitting at idle? That's a typical sign of a torn diaphragm.

Hahahaha!! It's true!!

I made some progress last weekend but not quite there yet. So I drilled out the two rivets at the switch. Replaced them with aluminum pop rivets for now. That made it better but it still didn't work. There was also a continuity issue at these other rivets The only problem is I used a grinder and as the rivet heats up it melts the housing. A die grinder might be a better choice. The LED lamp I was using worked for a moment then freaked out and wouldn't go full brightness. I ordered some more. So we'll see. I would like to replace the aluminum rivets with small copper rivets if I could find some good candidates and a proper way to staked them. They need to be about 0.095" or about 2-2.5mm OD

I think I would cap everything on the manifold that uses vacuum with an assortment of rubber caps. Then if you can get the engine started and sorted, add them back one at a time to check for vacuum leaks

Couple comments to hopefully help the process... So I don't know exactly how you are handling the choke lever, but the typical process is to give it a good pull, crank the motor, and then as soon as it starts, push the choke lever forward "some, but not all the way off". It's not unusual to need partial choke for several minutes to keep the engine running smooth when cold. If you're pushing the choke all the way off immediately as soon as the engine starts, you are probably moving too fast. Give it some time to warm up with partial choke before you try to get the engine to run with no choke at all. Engines don't like to run cold. and engines that have sit for decades are usually grumpy about being woke up from their sleep. It'll probably get better as you get more time running, but first bunch of minutes on an engine that hasn't run in a long time? Don't expect perfection. Next is there's no way you should need to be running five turns down on a stock motor. If you're thinking you need five turns down, there's something wrong somewhere. I'm hoping that it's as simple as you need to leave the choke on partially for five minutes, but if the engine is warm and you need five turns.... There's something wrong. And that "lifter" on the bottom of the carb does not make things richer on that carb, and in fact, it does the opposite. When you lift the piston like that, it completely disables the carb by making it super lean.

I haven't posted in a while.....hope life is treating everybody with kindness The single best thing I have done to my braking system was to install a larger 8" brake booster. Best of my memory 69-72's had a small 7" booster. Nissan updated the 240 brake boosters to a 8.5" in '73. They knew the boosters were to too small and corrected the braking problem. You also may want to inspect/replace your brake booster check valve. I did go a larger caliper but never got that great breaking feel until I completed this upgrade several years ago.......This made a HUGE DIFFERENCE in my braking power. JDM-Car-Parts use to have an upgrade for 69-72 that had the correct bolt pattern to accept into the firewall. That made the transition from a 7 to 8" a breeze for me with no extra drilling. https://jdm-car-parts.com/products/datsun-240z-1969-1972-8-inch-brake-booster-upgrade-kit?variant=31778739748973

I'm going to need to get some replacement floor pans as well. I have had at least three separate people tell me to go with KF Vintage. One was a contractor that had done previous classic car restorations, another was a vendor at the Carlisle Import & Performance Nationals, and another was a Z mechanic. Needless to say, that's who I will be going with when I finally pull the trigger and decide to replace my floors. Mine were filled in with old Maxwell House coffee cans and lots of filler by a previous owner that did not want to weld. I was planning on replacing my floors as this winter's project, but I may need to put it off due to the persistent Gov't shutdown. Not having a current and reliable source of income may stall my plans somewhat.

I wonder if the needles are set too low in the pistons? Closing off the fuel with the choke off

I wondered if it could be for air conditioning