I don't think that stock rubber lines are dangerous or anything like that. In modern times they're pretty foolproof. But the PTFE lines *ARE* stiffer when exposed to internal pressure. It's pretty easy to feel on a conventional braking system.
Some people act like it's the end all be all, but on the average vehicle driven on the street it offers no real improvement other than a very slightly (<5%) pedal feel/travel due to the stiffness change.
Just a FYI, the PTFE lines have a WORKING pressure of 3,000 psi or higher depending on size. Most stock automotive brakes will have a maximum pressure of about 1500-1700 psi, with the average panic stop being in the 1000-1200 psi range as a peak. Stock rubber and aramid woven lines cannot stand up to the same pressure as PTFE lines can, but they don't need to. But they're definitely not "the same" when it comes to capabilities.
I've personally taken Summitracing PTFE lines up to 11,000 psi in the -4 AN size for a few pressure cycles with no issues or visible deformation (testing out fighter jet hydraulic components). No way a rubber/aramid hose is standing up to that. Don't try this at home!
I totally agree with you there, they are an upgrade absolutely (I mean like I said my 4Runner will be getting them eventually) It's just, many people get this idea that because everyone things SS are the ultimate upgrade that their current brake lines are like aneurysms waiting to happen at the lightest stop. While true in say maybe a GM or Ford truck/SUV (seriously, why is it those Domestic trucks/SUVs have buttery soft brakes that always feel far too undersized for the vehicle? It's a bit disconcerting...), the system that is used in our vehicles (as well as a few other Toyotas) is designed fairly well, including the lines.
The hydraulically assisted master cylinder has a lot of assistive force when it comes to hydraulic pressure (part of the reason there's that notorious "soft" pedal feel that many people mistakenly believe to be the brake lines swelling and/or a "spongy/ineffective" pedal (not that it isn't a mild contributing factor.) If you've ever gotten bored and tripped the "Brake Assist Feature" going anywhere from 30~60mph in an empty parking lot or road, you'll know what I mean. When that system decides you need help to stop, it'll throw you through the window at the same speed you were travelling while the vehicle will stop a hell of a lot better than most any mid-sized hunk of steel with a frame I've had the pleasure of driving.
The factory service manual gives this as the "highest" testing value (i.e. this was the highest pedal effort given) while checking brake pressure with the booster pump active (vs. starting the testing by completely depressurizing the system):
At 196 N (20 kgf, 44 lbf) or pedal pressure:
Front Brake Pressure- 8,600 − 9,800 kPa (87.7 − 99.9 kgf/cm2, 1,247 − 1,421 psi)
Rear Brake Pressure- 11,310 − 12,510 kPa (115.3 − 127.6 kgf/cm2, 1,640 − 1,814 psi)
I used a bathroom scale to get a feel for what about 44lbs of "pressure" is by feeling how much I had to push the scale to get that reading; it's not at all scientifically accurate, but theoretically it should get me in the ballpark for something with as little resistance as the 4th gen brake pedal. There is a Brake Force Gauge somewhere in our tool room and I may look for it this weekend to play with it if I feel like it. At
about what 45~50-ish pounds of effort, the pedal was barely past the 2/3 (maybe touching close to 3/4 of the way down) mark on my vehicle; which if it's anywhere close to accurate indicates there's more pressure to be had if you wanted it. This is all "normal braking" by the way, I couldn't find any pressure readings for when the system goes into "Brake Assist Feature," which is activated by a combination of rapid & heavy pedal application (such as an emergency stop), based on Toyota's rough graph it looks like they are implying that when BA is activated the vehicle is outputting almost double peak pressure as compared to a normal application... that's a lot of pressure in the lines.
My point being that I don't think Toyota skimped on the design of the rubber brake lines if they designed the system to take about 1800psi normally and closer to 3000psi at an emergency stop situation. At least compared to a more traditional vacuum booster with older brake lines from back in the day.
Will Stainless Steel Lines improve stable brake pressure and hold up better due to being stiffer? Sure, that's why they are an upgrade; will they make your pedal feel stiffer? Unlikely you'll feel any significant difference in this department, you may feel the "braking point" (the point in brake pedal travel where the brakes are starting to bite into the rotor and engage) a little bit earlier and you might even feel a bit more crisp feedback through the system (the very slight expansion/rebound effects of rubber do create a dampening effect). Though it's not like your brakes are gonna get that "lead boot" level of stiff; the system's ability to assist the user and the way the fluid valves are set-up are meant to provide an easily depressed, but very consistent brake pedal (the difference here from being "spongey" or truly "soft" is that both these terms are used to describe an inefficient and/or weak brake pedal that is also not transmitting enough braking force (i.e. the pedal is soft and the car doesn't stop well vs. the pedal moves consistently almost the entire way with very little change in the starting pedal effort and the vehicle stops.)
Bottom line:
Stainless Steel Brake Lines are a good viable upgrade for our vehicles and do provide some benefits. Are they "required" mods or need to fix a serious issue with the braking system? Not really, if you have the money and time; go ahead and do it. If you're on the fence because you feel like you NEED SS Lines, I'd go put the money in some nice aftermarket brake pads and rotors if you really feel the vehicle isn't stopping well.
Post Note-
[MENTION=35264]RPS13[/MENTION] That sounds super fun about testing jet hydraulics, I'm jealous!
Another side note, all of our hybrids do not actually listen to you when you brake for the first 1/4~1/3 of pedal travel (no seriously, you're not doing anything hydraulically in terms of controlling the brakes.) The pedal feel you get in a hybrid in that range is simulated by closing off the port to the master cylinder and modulating a feedback to pedal pressure. The reason they do this is because the vehicle does a better job at recovering energy from "light braking" when it controls it than you would ever be able to do. Once you push the pedal further than that first bit, the port is opened and actual hydraulic force from the brake pedal is assisted to stop the vehicle.
Why do I mention this? Well two reasons, one, it's neat information to throw at your Prius loving friends that they've given up control of the brakes to the vehicle. Two, not all hydraulically assisted braking systems that Toyota made/uses have this simulator feature; the units used in the J120/J150 platforms (4Runner, Prado, FJ, Tacoma, GX, ect.) do no use a stroke simulator. So if you see any old posts by me about how the 4Runner has that, please send me a PM and I'll have to correct that errant misinformation. Thanks!
