Brake Fluid Explained

Hydraulic brakes are always used from approximately 7 mph to 0 mph, however regen provides impressive braking at speed. Enough to surprise the novice Toyota hybrid driver.

Yes but you quickly got used to it. It is all simulated in any case unless your brake booster system has failed. At that point you get a light show, no power braking and dramatically increased stopping distance. However Toyota does a decent job of predicting the situation before it happens and then alerting the driver.

The biggest problem with brake boosters in Toyota hybrids were early failures and no aftermarket options in gen2 and 3 Prii.

 

Darkness of the fluid and moisture content aren't correlatable, so hard to say using your eyes would be more reliable than a moisture sensor

 

Brake fluid turns black due to contamination, moisture absorption, and thermal breakdown.

I suppose that when my brake fluid goes from honey coloured to USN Chief coffee color I could spend $15 on test strips and open up the system to check things out.
Or?
I could just spend the $15 to replace the fluid in the system and get on with my life.

It's like getting your engine oil analyzed for $50 rather than spending that money to just change the oil.


MY MILEAGE.
Yours may vary.

 

Mostly due to copper absorption not moisture level, as in the video. Your fluid may still be safe another 6 mos, despite being dark. That's quite a margin for guesswork, was my point. But if the work isn't a burden to you, do it every other year -- done properly, can only be more dependable for best brake performance, all else being the same.

Moisture sensing tools make the distinction unambiguous, and cost less than the quart of NAPA / Gunk / Prestone / Lucas that may suffice for a typical full brake fluid swap + bleed. One would be petulant not to spend the pennies to use one (as full brake fluid swaps are enough work to avoid being done too early... esp given if done wrong, can be pretty expensive on a few levels)

If moisture level made brake fluid dark... doesn't explain why my old 150K '04 Honda spent 5y with me, in 75 - 100%-humidity and never got even a bit darker. And I defo used the brakes into ABS regularly on that 7th-generation piece of junk, so there's thermal degradation out the window (sue me, it was a manual in a Honda... heel-toe-blipped downshifts braking late going in, and the on-throttle mid-corner behavior coming out, were some of the only pleasures in that heap)

Wearing out two sets of NAPA front pads and one set of shoes in 5y was probably asking for trouble... but up until now, were depending on the same rule of thumb as you do (that vintage of Honda must not have used many copper crush washers or other components touching brake fluid) -- never again

 

Copper contamination is.......contamination.
Honestly, this is one of those BITOG debates which is pointless in a Prius forum.

Most of today's drivers have never even CHECKED their brake fluid, let alone changed it.
One of the reasons I have over 200,000 miles in Priuses but have not owned a Toyota since the1980s is their corporate culture which tolerates a dealer network that encourages a "closed hood" maintenance philosophy.

Frankly?
"They all do it."
The only difference is that I have more access to GMC and other LOCAL dealers and so I don't have to pay the "Toyota Tax."
I buy new, relatively base model vehicles and pay them off as quickly as I can.
That's right.
I still make payments on cars despite frequently telling people that it's usually moronic to do so - but only because of the fact that money in the market is 'worth more' than the amount of interest paid on a short term auto loan which is currently about 4.5%.

 

Just checked -- there's nothing on a 5G preventing a brake fluid swap. Same bleed nipples, same reservoir under the hood. Need only basic tools and tolerance for properly wiping up and degreasing the inevitable smol mess...

Should be even easier on a domestic. Ignore your brake fluid moisture level at your peril -- if there's one system on any motorised vehicle I test before setting off, it's brakes (even now with my 5G, every morning). Can't test for heat-related brake failure from steam forming in your fluid in the car park... so just swap every couple of years in a humid climate -- no sooner, but defo no later. Sensor will hugely help with exactly when, independent of fluid colour

 

Repair Manual concurs; see attachment to post #17.

 

Swap and flush often get interchanged in these discussions, and lead to confusion. A flush requires special software to open all the ABS values to access those lines. A flush is not a regular maintenance item.

 

I'd say the main terms getting confused are the ones Toyota uses, namely "fluid replacement" and "air bleeding".

Because the system has some electronic valves in it, there are always places you're not getting if you follow any procedure that doesn't have a scan tool getting the right valves to open at the right times.

For "fluid replacement" that's not much of a problem. So you've replaced 99% of the fluid and there are some traces of old fluid left that get all diluted and mixed with new as you go on driving and using the brakes.

For "air bleeding" it's unacceptable to leave any passages unbled, because it's not acceptable to have bubbles in the system anywhere. So that's why, in some generations anyway, Toyota repair manuals offer a simpler procedure for "fluid replacement", but require a scan tool for "air bleeding".

"Flush" isn't a distinct term Toyota uses. The way you used it above, it sounds like a "fluid replacement" where you're really not willing to leave even small amounts of old fluid behind. So, you'd just do it the same way as "air bleeding".

 

Which are clear terms, but rarely used in lay discussions such as these. I've seen bleed used for decades in such for a fluid replacement. Likely stemming from back before electronic brake control and the two procedures were the same thing.

 

Toyota Canada's words, in the 2014 Prius Owner's Manual:



I've seen the term "flush" in Honda Shop Manuals, applied to replacement of transmission fluid, when there's been contamination of some sort. It's multiple replacments with a drive after each.

 

And sometimes the lay usage creates comprehension problems, of which this is a frequent example on PriusChat, because "fluid replacement" and "air bleeding" are the names Toyota uses, and there's one you can do without a scan tool and one you can't, and the countless PriusChat threads rehashing "can't I do that without a scan tool?" keep springing up because one gets mixed up with the other

 

I'm not sure how much that's a thing in modern ABS. The Prius actuators include linear solenoids and can just modulate the pressure to the point just back from loss of traction.

I don't think the answer is any more complicated than the question. You've already described what we what the system to do, so the ECUs do that.

There's even a little bit more to the behavior than that. At high road speeds, regen would quickly produce more power than the battery can accept, so the ECUs will only produce as much regen force as it can afford, and will use friction immediately for an the rest of the braking demand. As the road speed comes down, more and more regen force is affordable, trading off for friction. Then, once down to single-digit speeds where regen can't generate as much force anymore, it gets traded back away for friction.



The ECUs can easily do all the math involved in those decisions. ECUs are good at math. Then they just control the inverter to apply the computed regen force, and control the linear solenoids in the brake actuator to apply the computed braking force. It's impressive engineering but not conceptually hard.

Your pressure on the brake pedal, except in failsafe mode, isn't going toward the calipers at all, but only into a springy "stroke simulator". The fluid pressure actually going to the brakes is just what the ECU wants it to be.

 

As I understand it currently... ABS systems modulate pressure to the caliper in order to control brake lockup. Which means 1) full, unadulterated pressure needs to be applied then monitored by wheel speed sensors, until lockup (or onset of lockup in more modern systems w/ faster sample times from mulitple wheels, I assume). Then the system has to relieve brake pressure until lockup (or some phase of lockup) is relieved, whereupon the system will continue to monitor and reapply brake force, until the car has stopped. So even w/ current systems, some pulsing in inevitable.

But that can be compensated for with things like small tuned hydraulic dampers -- same coil-sprung sealed volumes used in civil engineering to prevent hydraulic shock damage in water pipes. Coil springs and metallurgy these days is such that these can be tuned to only work under conditions that can damage the pistons or other components, yet still provide decades of reliable, consistent braking. Just a guess, tho...

The last paragraph, 'your pressure on the brake pedal, except in failsafe mode, isn't going toward the calipers at all, but only into a springy "stroke simulator". The fluid pressure actually going to the brakes is just what the ECU wants it to be.' is similar to what I'd mentioned earlier as 'regen' doing the slowing initially, then hydraulic ('failsafe') calipers proving more -- under ABS control -- if needed.

What I didn't understand, was the speed function part in regen, basically under what conditions braking is happening -- even intuited there's no way regen can do anything at the walking speeds rolling to a stop, so must be all-hydraulic there.

Looking at the charts, makes perfect sense. Just didn't put two and two together without them