Captain Obvious

What he said. Just cap off the brake booster and the transmission vacuum lines for now. As for the intake/exhaust gasket, yes they are cheap and readily available. Only problem is in order to replace it, you need to pull both the intake and the exhaust off the head. And that extra trouble step is why sometimes people try to reuse the old gasket. If yours was in good enough condition to maybe reuse, then some silicone would probably make it leak free. But if half of your old gasket peeled off the head and stayed with you original flat top intake manifolds.... Well then, you're asking for trouble.

Oh, that was no accident. That was desperation and frustration right there!! Cutting wheel on a 4 1/2 angle grinder, and then a BFH to crack through the center portion that I only cut through partway (so I didn't cut into the axle flanges).

Glad you got it off. Now that it's off, can you tell why it wouldn't come off in the first place? In the past, I've encountered a thin "unswept" ridge of friction surface on the inside of the drums that would catch the edge of the brake shoes. Is that what was happening with yours?

Oh, and I don't see anything else glaringly wrong in the pic, but was wondering... When the flat top to round top conversion was done, did they (you?) use a new intake/exhaust manifold gasket, or reuse the old one?

Well let's hope fixing those two plugs helps your running issue. If the two 10mm head bolts were screwed in somewhat tight, they probably sealed "mostly". If those bolts are the only vacuum leak you found, they probably weren't large leaks.

Lol!! The Yarbinator. Love it! ☺️

Tapered pipe threads are supposed to tighten up on the threads, not the underside of the head. In fact, if the head were to bottom out, the threads would run the risk of not being tight enough to seal. In other words... The hex headed plugs you have there are working exactly like they should. You would stand a better chance of achieving your "clean look" with the recessed hex head "Allen screw" type plugs.

I found some good pics, etc. of that booster here >> https://irate4x4.com/threads/ibooster-electric-brake-booster.394316/ Mounting: The mounting bolt pattern is 2.375" x 3.1875" square with a 2.5" center bore pass through:

Yeah, hopefully it should be easy to tell. In any event, it's got to be better than this!!! Hahaha!

KYB is pretty specific about using no oil, but many people (including me) defy that recommendation and use some anyway. Since you aren't using the oil primarily for lubrication, but instead, more for rust prevention, I would recommend an R&O oil. That means "Rust and Oxidation". R&O oils have additives intended to minimize R&O. Here's a couple links for info: https://www.valvolineglobal.com/en/r-and-o-oils/ https://www.phillips66lubricants.com/product/multipurpose-r-o-oil-mining/

Do you have any more info for this? Where did you see it?

I believe that all the tapered fittings are BPT. Probably dictated by world manufacturing climate at the time. Some of the BPT and their respective NPT counterparts are different enough to easily tell the difference, and some of them are really similar. For example, with some thread sealant, it usually seals if you stuff a 1/8 NPT plug into a hole threaded for 1/8 BPT. But, I think the BPT is correct. There also does exist some locations where a straight thread was used (temp gauge sender unit for example), but I've found them to be straight up metric thread, not BSPP.

Well, according to the info Zed Head posted above, the longer spring goes on the RH side.

Sigh...

When I do the math on the free lengths from the manual, I get significantly different numbers. 373.5 / 25.4 = 14.7 386 / 25.4 = 15.2 So assuming the free lengths in the FSM are typos, the lengths you are measuring look great.

I wasn't there when they made the exhaust manifolds, but my assumption would be the same. I would expect they were raw uncoated cast iron. Maybe a thin coat of oil so they didn't flash rust in the parts bin, but that would burn off in the first few minutes of engine running.

Glad to help. Hope you make good progress and get things working well soon!

Some thoughts on the above. First, the 1/8 pipe thread specs are very similar between NPT and BPT and they would be difficult to differentiate. The 1/8 NPT spec is 27 threads per inch, while the BPT uses 28 TPI. If you are holding a fitting up to a tap and that fitting only has four or five threads on it (which would be typical), it would be very difficult to tell the difference between 27 and 28 TPI. Second, there were no (what are typically called) "barb fittings" used by Datsun on the balance tube. So if you had something like the below installed, it was a previous owner modification. This is what I think of being called a "barb fitting": So if you had something like that in the 260, it WAS probably 1/8 NPT, but it didn't come from the factory like that. ☺️

Ewwwwwww. They coated the boards. "Conformal coating" it's called. Looks like every place the coating wasn't perfect, the metal underneath corroded. So the P.O. commuted underwater to get to work every day?? ☺️

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.

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.

Hahahaha!! It's true!!

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.