This is in no way a criticism of your post/thread, but I think the thing that keeps giving me pause on trying this is the amount of adapters and new points of leaks/failure it introduces. I would think using something like this:
https://empius.com/products/2-psi-residual-pressure-valve-sae/
would introduce the least amount of new connections, and also uses an SAE flare fitting instead of NPT + thread sealant (I've always been told not to use thread sealant on brakes in case of contamination). Only issue would be having to cut the hard lines to install that fitting on the front circuit of the master cylinder.
Also, when you initially installed the RPVs on the master cylinder, and then went to the wheel well setup, did you end up replacing the hard lines?
No worries I don't mind questions or new ideas, and you are not the first person to bring this up. I guess it is because I have done so many projects like this (in some cases practically building an entire brake system from scratch), that this seems like a relatively minor low risk modification to me. NPT fittings with sealant like this are frequently used in custom/aftermarket brake systems, which is why they are available for purchase all over the place. These NPT fittings with this sealant create a seal that will withstand chemical exposure and pressures far beyond what it will ever see in this brake system, and it is literally made for this type of application.
There is no reason to be concerned about using them, sure it may not look as clean, and if you were to get significant quantities of this sealant into the system that could be a problem, but if you apply it as directed (see pics), that cannot happen. You are supposed to apply it a couple of threads down and then let it harden before assembling the parts. If done that way it will not cause any contamination issues and will provide a very safe reliable connection.
"Permatex high-temperature thread sealant is an OEM-specified product designed to lock and seal threaded fittings on a range of applications. It prevents leakage from vibration, temperature cycling, and extreme pressures—and it won't shred or tear like PTFE tape. This high-temperature thread sealant is exclusively formulated to
withstand up to 10,000 psi for fast, responsive curing on metal, tapered pipe thread fittings. The PTFE offers good antifriction properties and high resistance to chemicals and harsh environments."
"
Suggested Applications: Head bolts into through holes, oil PSI sending units and sensors, oil and coolant lines, fuel fittings, rear axle fill plug,
brake,
and power steering fittings"
So unless there is corrosion, or you severely over tighten a fitting so that it starts to gall or crack, these fittings are as strong and reliable (if not more so) than any other part in the stock system. In fact the sealant acts as a lubricant while assembling, so it provides a great seal without having to excessively tighten the fittings. It also acts like a mild glue making the possibility of the fitting loosening up or develop a leak over time due to vibration extremely unlikely, perhaps even less so than the stock compression fittings.
Some even say that bubble fair fittings are supposed to be one time use fitting, which is absurd IMO as they are frequently reused multiple times by virtually everyone. However, if you over tighten them they can get distorted and cause leaks. But if you are careful and check for leaks there no need for concern.
In my experience brake lines/fittings do not spontaneously develop leaks unless they were not tightened properly, or there is severe corrosion, or damage from severely over tightening (which the sealant makes less likely), or some other damage like a rock hitting a line. The weakest link within any brake system is always the flexible lines, so if you are not worried about those, why would you worry over a couple of added fittings? As with everything, if you install them properly there really is no need for concern with these RPV's or the fittings.
I just use moderate torque, bleed the system, clean everything up, then start the engine and stand on the brake pedal to check for leaks. If you find one give it 1/8 incremental turns until it stops. When there are no more leaks you are good to go. It really is no less concerning than any other brake work/modifications, as long as you know what you are doing, and if not, find someone that does to help you.
Second question: I did not cut or replace any hard lines. When I was doing this I had no idea if it would work, so I made custom lines that let me install the RPV's without modifying the stock hard lines (see pic on page one). The brake system in our cars use what is called a "dual diagonal brake circuit" meaning each line off the MC feeds one front and the opposite side rear caliper. So if you want to install these RPV's under the hood for just the fronts, you would have to insert them in the lines coming off of the ABS unit, and there is not a lot of room to work with up there so... Putting them in the wheel wells is a much easier proposition.
My flare tool is the type designed to work in a vice, I don't have one of those nice portable flare tools, so flaring a line on a car is not something I am setup to do. However, now that I know it works, and if I had one of those nice portable flair tools would I consider cutting the line and inserting the RPV inline further up. Sure why not, but I think most folks here are already intimidated by a couple of fittings, so probably not a good option for most folks . However, now that I already have it installed with the fittings, I don't see any reason to switch it up now. It will look a lot cleaner when I look at it with the wheel off, and I know that kind of thing is important to some folks on here.
I certainly would not fault anyone that wants to go that route. Maybe in Europe you can find a 2 PSI RPV that has the flare fitting built in, that would be a really clean install. There are a lot of different ways to get this done and I encourage anyone to install them in whatever way works best for them. Good luck!