Prius 4th GEN Brake diagram

At first glance, I look at it and thing, I don't know, it's about the same part count, dozen solenoid valves (ok, Gen 3 has 14, so they saved a couple), now shuffled out into a couple different boxes.

But then I see that change in where the four wheel outlet valves lead. In Gen 3, it is straight back to the reservoir at atmospheric pressure. In Gen 4 it is just to those local accumulators and the return pumps. So even if one of those outlet valves gets leaky, there's nowhere much to leak to, unless the return pump activates.

Still, I wonder which valve(s) are really doing the leaking in Gen 3s that get the won't-hold-pressure problem. I'm surprised with the number of people dealing with that, nobody posts about taking the actuator apart to see which valve was dodgy. Because you hear the bad ones steadily losing pressure while just sitting there with the brakes not applied, I suspect the culprit is one of the valves in the upstream, pressure-controlling portion ... which is, pretty much, still here.

 

I am limiting my "reliable" commentary to the downstream actuator assembly- which by its design is inactive for 99% the time. Might see one or two failures a year out 2-3000 cars. If you do have a seeping outlet valve, then (on a conventional car) you have a soft- sinking brake pedal. On this Prius I would expect to have some sort of "low pressure in xx hydraulic circuit" code.

I would think that even if this part number is specific to the gen4, it would be much closer to normal car designs and would likely be produced on the same manufacturing line as others

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Looking more closely at the fancy "upstream" brake actuator, it doesn't seem to look like the gen2 or gen3 at all. Both of those basically had high pressure fluid from the accumulator used to directly apply the wheel end brakes. This gen4 appears to use high pressure fluid to move the master cylinder pistons and thus apply the wheel end brakes. Don't see any other fluid path for the RH-F and LH-R brake circuit to build pressure.

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Yeah, in the Gen 3 diagram you have the SCC valve that opens whenever the brake ECU feels healthy, and joins the two downstream pairs so they both receive the fluid applied through SLA and released through SLR.

(SMC and SRC are shown here in their fail-safe, open position; the ECU closes them when it is controlling the brakes. I just recently figured out, via Techstream, that it leaves those two open nearly all the time and closes them only when you begin to apply the pedal; that's the source of the click you can hear in Gen 3 at the very top of a pedal press.)

vvv Gen 3 Gen 3 Gen 3 vvv

^^^ Gen 3 Gen 3 Gen 3 ^^^

Gen 4 has clearly done away with that valve, so the RF/LR wheel pair is a truly isolated diagonal operated by the front M/C piston. That's gone back to what the big new selling point was when I was a kid; I remember car makers touting their "dual-diagonal independent brakes" like it was the cat's meow.

They have been using that principle to boost the brakes in every generation except Gen 2; the fluid flows into the "brake booster" chamber at the back of the M/C and adds to your own force pushing the piston, the same way a vacuum booster would if the car relied on vacuum.

In Gen 3 during the first stage of fail-safe operation (when you still have stored accumulator pressure, but the ECU has recused itself), which is the condition shown in the Gen 3 diagram above, the servo-regulated boost pressure is not only used in the M/C booster but also goes to the rear wheel pair (but not the front, because SCC is closed). Of course that same hydraulic pressure is acting on the M/C piston, which means the fluid in the front chamber is feeling it too, and that's how the pressure is applied to the front pair.

So Gen 4 doesn't look all that different really, except the pairs are diagonal rather than front/rear, and there's no SCC that can open to connect them. And the servo pressure regulator has been moved out of the skinny end of the master cylinder to be a separate part inside the upstream actuator.

The way the Gen 4 diagram has drawn the SLA/SLR connections to the servo regulator and the LF/RR pair makes me go crosseyed a little. I still haven't signed in to TIS and just downloaded the whole brake section out of the NCF manual, but I'm definitely curious enough to do that as soon as I have enough reasons to sign in.

 

Oooohhhhh. Nows I understands. I was trying to put everything in perspective from the "fully assimilated by the Borg cube" design of the Gen2 that I am familiar with. Gen3 and 4 ain't none such thing.

The Gen4 is in essence a conventional braking system with an integral hydroboost and non integral ABS unit. Toyota just put some "voodoo magic" between the brake pedal and the booster. Normal braking has the pedal "connected to" the stroke simulator, and its pressure sensor is the input to the ecu to signal braking request. If the ecu decides to apply friction brakes it activates SLA / SLR to control fluid through the regulator to the servo (aka- the M/C booster). The regulator does modulate booster pressure with the two linear valves, but it mostly seems to be there as a back up to apply and control the booster pressure if the ecu goes offline.

So the ecu applies the booster/servo to move the M/C pistons and actuate the hydraulic brakes. If ABS or VSC functions are needed then the downstream actuator does its thing. One of the fail safe modes has the SGH close (which traps fluid behind the M/C pedal input piston and "connects" it to the booster piston. SSA is open which dumps pressure from the simulator, and the linear valves default to no modulation, so boost/servo pressure is controlled by the regulator for as long as the accumulator has pressure. When that's done you're left with muscle power to operate all 4 wheel brakes.

 

I keep looking back at that diagram and thinking "now why didn't I notice that before?".

The servo-regulator in Gen 1 and Gen 3 was located in the tip of the master cylinder. That meant they never had room in the M/C for two genuinely isolated outputs. They had one isolated output going to the front brakes, and for the rear brakes they shared the servo circuit feeding the boost chamber.

Moving the regulator out of the M/C gave them room for a second isolating piston in there, so now both diagonal-pair outputs come from those, and the servo output only goes to the boost chamber to push on them. (Or am I wrong? The connections around the servo regulator in the drawing are still making me crosseyed.)

It is funny how different Gen 2 was. Gens 1, 3, and 4 are all kind of recognizably incremental evolutions of the same idea, while Gen 2 was an odd six-year feint off in a whole different direction, with no M/C booster, full brake-by-wire, and a box of capacitors to keep the brake-by-wire alive.

Gen 1 had only three-channel ABS (front left, front right, rears together), and the stroke simulator cut valve was overlooked (so in fail-safe, you had to bottom out the simulator before what was left of your leg power got to the wheels). Gen 3 was the same basic thing with a fourth ABS channel and the simulator cut valve. Gen 4 takes that, moves the regulator out of the M/C, adds a second piston-isolated M/C output, and moves the ABS into a more-generic downstream component (even though said component still seems to only be used in Prius).

Oh, and Gen 1 still had a traditional hydro-mechanical proportioning valve to deliver the necessary lower pressure to the rears than the fronts. Every later gen has been EBD (electronic brake-force distribution). It looks as if they must be using the ABS actuator for that purpose as well, with nothing else in the diagram that could do it. The two outputs from the upstream box have the same pressure (and anyway they are diagonal-pair outputs now, not front vs. rear).

They could close SRLH and SRRH to hold the rear pressure at the intended fraction of the front pressure. But they haven't got pressure sensors there for feedback. I wonder if they have just allowed ABS to subsume the EBD function ... "if the rears start to lock, you've found the limit pressure."

 

The servo piston appears to be the "only" thing that normally pushes against the primary and secondary pistons in the master cylinder.

Look at the regulator this way. The right half is a 3 port shuttle valve (it can apply or release pressure to the servo). The left half has 2 pistons that control the valve. Pressure (from the M/C primary circuit for the outer piston, or the linear valves for the inner piston) pushes each piston to the right and springs push left.

Push the shuttle valve to the right and accumulator circuit is connected to servo circuit and servo-boost pressure goes up (apply). Let the valve go to the left and servo circuit is connected to the return line so boost goes down (release). Modulate the valve and you get any pressure in range that you want.

The linear valves must have more authority over servo pressure than the primary M/C circuit piston, otherwise you would have a "runaway" brake apply. (The ecu applies the brakes, which increased the primary pressure, which increased servo pressure and applied the M/C more, etc)
Ahh- just thought about it and pressure from the linear valves counteracts pressure from the M/C piston.

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