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Thanks for the help, Dave.

Just so I understand. The brass fitting is pressed into the end of the master cylinder? And the stoplight switch is steel?

So I would remove the old switch and drain out the DOT 4 brake fluid.

Should I attempt to clean the master cylinder threads by gently retapping them?

Then spray it clean from the inside of the master cylinder with brake cleaner. Then install the new switch without thread lock. Right?

Here is a link to a video of what was happening before I took it to the Oregon muscle car shop.
mktx.com/packard/MasterBrakeSwitchLeak.mp4

No, the brass fitting is not pressed onto the MC, it's either threaded on or held in place by special brass washers and a special bolt, normally the latter though I can't speak to your specific model.

To simply change switches, it is not necessary to change or drain any fluid, and often it's not even necessary to bleed the system after installation.

The thread of the switch and fitting is an NPT thread, not a conventional thread. So if it is damaged, replace the fitting or bolt, do not attempt rethreading even if you have a proper tap.

Don't use any fluids that might leave a residue and contaminate the brake fluid and alter it's properties. If you want to clean the brass fitting to inspect it, remove it from the MC first. And make sure it's clean and dry before reinstalling.

Wow, the plot thickens.

To determine the size thread on the stoplight switch, rather than open the new package, I dug up an old original switch I'd saved and tried to match that with a die. It looks like 7/16 - 20, but I didn't twist it on because there was a little resistance. I was going to take it to the hardware store and screw it into a sizing jig, but that's a mute question now, since it's a special thread, they probably don't have and rethreading is not a kosher option.

On closer inspection of the new stoplight switch, I compared it to the old one I'd saved. The new one has female connectors and the original has male.

My Harnesses Unlimited harness wants to connect to a stoplight switch with male connectors, so I'll have to call Kanter and return this one with female connectors. The new switch with female connectors is stamped 06 07263. The old (maybe original) switch with male connectors is stamped 08 16215.

Who can you trust? I might try getting a new one from NAPA since I'll have it in a day instead of a week+ and I need my car.

Next I'll have to check and source that fitting on the MC. Hopefully Kanter has it.

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When doing the 47 brakes several years ago I too had the issue of a drop or two leaking each time brakes were applied as apparently have a few others -- but not all -- when using the Napa SL134 switch. Could you check how far your new switch threaded into the bolt. I just looked at mine which is the Napa SL134 replacement and see it threaded in almost to the shoulder of the switch. Out of curiosity I tried an old original but bad switch I have on a spare bolt, making sure the threads were clean on both, and it went about 5 or 6 threads in before starting to get tight. Am almost wondering if the Napa switches actually are 1/8-27 straight thread vs 1/8-27 NPT or if they are using a one size fits all approach with a minimal NPT taper that might be made with a looser spec and doesn't work well with older fittings that were true to US spec..

Am not sure where the brass O_D mentioned is coming from. My output bolt on the 47 master is all steel and so is the spare bolt and the switch thread. I will be the first to agree with O_D any kind of sealant should not be needed and is a practice to be avoided but sometimes need forces bad behavior. I used teflon tape and that seems to have solved the problem for now. If you are going to try some kind of liquid sealant, IMO Permatex #1 (hardening) or #2 (non hardening) or one of the thicker pipe thread sealants impervious to brake fluid might be a better choice than a thin thread sealant. Admittedly my car has just sat in the garage so the teflon tape has been unproven as a permanent fix but it has held up so far and the leak is still not present which was all I wanted at the time.

If you have another SL134 and some calipers maybe you could give a quick check to see if the switch is actually NPT. This old switch has a very very slight taper and difficult to see and more so to photograph but holding up so a light could shine thru from behind I was able to visualize a slight difference in diameter on the original switch. Don't have another SL134 to check and don't want to remove mine from the car but would be curious if you or someone else could check and see a taper.

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If someone has difficulty finding a NOS switch with female terminals or it is prohibitively priced you could change the male terminal now on the wire to a .156 female bullet terminal available at most parts stores and use the modern switch.

If you want wires to stay stock then use a .156 bullet 2 way connector to bridge the new vs. old. One end of the 2 way connector would go on the male terminals of new switch and the old male terminals on the wires could go in the other end of the 2 way connector.

No idea if the 2 ways are sold in small quantity at parts stores like the female terminals but Amazon has them in bulk.

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Thanks, Howard, this helps my understanding.
My pix are fuzzy, but you can see that the brass fitting looks a little thrashed. The shop may have added a copper washer to allow for more tightening and some flattening of the surfaces.

The taper on the old (maybe original) switch is barely perceptible. I'll check the dead NAPA switch when I take it out. I'm asking Kanter for the brass fitting.

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To use the switch from Kanter, I would need a connector with a bullet on one end and female terminal on the other. The female end would slip over the male switch terminals and and the male end would plug into the two female wire terminals. .156 is what I'd look for. It could save rigmarole. Thanks.

NPT threads are determined by the nominal inside pipe diameter and the thread pitch so, as stated, the switch is 1/8-27 NPT. The brass block, though, is typically not a standard item and was made specifically for this application so if your threads are worn, best to look for an NOS or new reproduction. With decades of tightening in the new senders, the brass stretches little by little until it gets to the point that the sender bottoms out before a seal has been made.

A valid point for any switch that threads into the brass block. While I can't speak to any of the thirties senior models that might have used hydraulic switches, I believe prior to 51 the majority of brake switches on the other models, rather than thread into the brass block, threaded into the end of the steel bolt directly out of the master that holds the brass block.

Joe has a photo of his master and switch a few posts above and my 47 is the same.

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I have an email into Kanter. They say they prefer email communications. I’m asking if they have the fittings that go between the MC and the switch, both of which I purchased from them.

Gosh I’m missing something. The die closest in size that twists on to the switch is 7/16”. I don’t get the reference to 1/8”. My mechanical pea brain strikes again.