Stuart - Thanks for the tips / advice.
Right sensor? Who knows! There were a few different ones available. All $300+ USD. Pretty crazy if I do say so. Also, I did use the LeafSpy application to clear the errors. However, with the system in somewhat permanent error, the cleared codes would return immediately - the warning lights wouldn't even go out.
I can't confirm it was an exact match to the original - didn't want to pull the original and have a mess of brake fluid on my hands and also didn't want to install a $300+ part that might not be the source of my issue. I read a lot of the service manual dealing with the master cylinder pressure sensor, and never was able to locate any specific details about what the resistance measurement should be on a properly functioning sensor. I find that to be bizarre. Maybe I just wasn't looking in the right place. The other thing that I was curious about is, given the sensor's job is to ensure there's pressure in the master cylinder, maybe it should be throwing an error in the event there isn't (atmospheric only) any pressure. So I Mcguyvered up a hose to hook to the end of the new sensor, and with a bike pump, held around 25psi of pressure (I have no idea what the master cylinder pressure is) in hopes that with pressure applied to the sensor, the error would go away. Alas, even with the pressure applied (and wiring lead connected) the car still showed an error with this "new" sensor.
My assumption is that like most sensors, there's no logic in the sensor itself. It has a specified range of operation, tailored to the ECU scaling of the Leaf. So for a set amount of pressure, a corresponding resistance results in some variable voltage between 0 and 5v. I did do an analysis of my aftermarket sensor. But since the stock sensor was never removed from the car, and access wasn't available, I wasn't able to compare to stock.
For those that might come across this in the future, here is my best at explaining the sensor and the results:
The sensor has 3 prongs. If you orient the sensor so that the prongs are facing you, clock the sensor so that the catch/clip on which the wire connector clips is in the 3 o'clock position. In this position there are 2 prongs on top and one centered below them. Going back to a clock, let's say they're in 11 oclock and 1 oclock up top and 6 oclock on the bottom. Heretofore, referred to as "11", "1" and "6"
11"-"1" = 6k ohms
"11"-"6" = Open Loop
"1"-6" = 181k ohms
So I had been dealing with a persistent brake master cylinder error (as mentioned above) for many days on end, if not a period of a couple of weeks before ordering and attempting the sensor swap. Before that these errors had been an exception, not the norm, and a restart of the car would reset it. Suddenly, last week during a little downpour, the errors went away. I was thrilled, briefly. As the errors returned sporadically throughout my driving that evening as well over the next few days. At first I was thinking - well with the car getting wet all over, maybe I have a grounding issue somewhere? It just isn't adding up. This behavior continued. Often the car would start without the fault, but upon driving even 50 yards, or 2 minutes later, the fault would come, often when I used the brakes.
One thing I have also read about is how the brake fluid itself, being stale and cruddy, could contribute to the problem. So I sucked as much as I could out of the master cylinder (maybe 3 - 4 oz?), which frankly wasn't much as there's a partition about half way between the low and full mark whereby I couldn't snake even a small diameter hose past. I replaced with fresh fluid. Drove the car the next day, no lights. I checked master cylinder again and the fluid had mixed with the other and so it all look kind of maple-syrup like again. Since I needed to rotate the tires anyway, I threw the car up on jackstands and cracked open the bleeder valve on the front left wheel and let the bleeder start oozing fluid. After about an hour I got impatient and (being careful to keep the reservoir topped off with fresh fluid) used the brake pedal to manually drive the old fluid from the master cylinder. I pulled probably 20 oz of fluid out, topping the master back off with fresh along the way.
I've driven the car probably 50 miles since the service, and have not had the lights return.
Also noteworthy, I've put around 30K miles on this car since I purchased it 3 years back, and it now has a bit over 60k miles on it. During that time, I always felt the transition between regen and braking to be less than smooth. It required a careful / mindful foot to ensure consistent negative acceleration forces during stops. With the fresh fluid, this feels much much better.
Since I was bleeding the system alone, I was worried I might get a little air in the line. Brakes feel no different. If anything a little better, but may be placebo effect.
Ideally, I would have done all 4 calipers, as I recognize old fluid will still be in the lines and calipers of the other 3 brakes. But it was hot and I was short on time and really just trying to see if this exchange of the majority of the fluid would net positive results.
I'll try to come back and update this thread. I'll honestly be a little surprised if this is the solve for the problem I've been facing. Just seems a little too easy and I can't rationalize why old fluid would cause the system to error. But at the same time I cannot deny the difference in the transition between regen and braking, so maybe there's something to it...