Power brakes went out while stopped with foot on pedal.

Some meters will have a REL/Δ button you can use to memorize the meter lead resistance and show it as zero, and then show the resistance of your test resistor relative to that. I guess your 179 doesn't, but the old-timey "write it down then subtract" approach also works. Maybe not a substitute for a four-wire low-ohms measurement, but usable in a pinch.

Then again, in my experience, the meter lead probe tips have to be really clean for doing that, or even the initial lead-resistance measurement jumps all over the place.

 

I can't understand why you keep asking for help from trolls on the internet who only have anecdotal* personal experiences - but you also don't believe two dealer's with the same serious diagnosis.

One of my anecdotal personal experiences is there are no semi-reconditioned parts and any dealer will verify part / vin compatibility, including some online. I would not fool around delaying this repair - emergency rooms are much more expensive. Just a personal *... you know, so please disregard.


* not necessarily true or reliable, because based on personal accounts rather than facts or research

 

This thread seems to be running on a couple parallel tracks: one about which 12 volt batteries are good or not, and one about the OP's brake issue.

I would judge the brake issue to be the more pressing of the two.

The OP did go to two dealers, both of whom proposed to replace the brake actuator. At the same time, the first dealer got there by ignoring what the trouble code reveals about what the problem is, and the second dealer got there by looking a little harder at the code, and then offering a technobabble made-up "explanation" for how "inflow and outflow cycles of brake fluid" create an electrical short to +12 on the control side of a solenoid valve.

Those are both ways a dealership can earn some skepticism, even from a customer who went in willing to trust them completely.

When it's at high-dollar, high-volume shops like a dealership, I think a big part of the reason is just that it is fussy, time-consuming work that ties up a high-rent service bay. Also, they probably know exactly which of their techs are any good at that kind of work, and they probably spread those techs pretty thin.

Compared to indy shops, the dealerships are also used to a clientele more willing to sign bigger checks without hard questions, and so a repair strategy of "swap X expensive part on the customer's dime, if that solves it great, if not swap Y expensive part on the customer's dime, if that doesn't solve it either then get Tom on the job to find out what the problem is (on the customer's dime) and fix that" can work for them.

 

What you put in quotes there, "decreasing pressure Solenoid FL", is what we call the "fortune cookie" for the code.

A code's fortune cookie is not what the code "means". What a code "means" is its "detection condition", which you see printed in a box at the front of the code's workup section. The detection condition for C1357 INF 28 is:

When you try too hard to torture meaning out of the fortune cookie, it is easy to get twisted around. "Decreasing pressure solenoid FL" doesn't mean there's some solenoid, and the ECU is telling you the pressure decreased.

The code is about SLRFL, which is the "pressure decreasing solenoid FL", that's its name. It is a valve that opens when you send it voltage, and makes the FL pressure decrease. That's its job. (There is also an SLAFL, which is on the pressure supply side, and opens when you send voltage to it and makes the FL pressure increase. And there's an SLAFR and SLRFR, and SLARR and SLRRR, and so on.)

The code you have is not the ECU detecting a drop in pressure on the front left. The code is the ECU detecting voltage on the control wiring to the valve. That's the wiring that carries the ECU's control signal to the valve, and the ECU also contains a voltmeter on that circuit so it can detect weird things, like the voltage not going up when the ECU tries to turn it on, or the case you have, with voltage showing up on that wire from somewhere else when the ECU is not trying to turn it on.

And when extraneous voltage appears on the control circuit to the SLRFL valve, of course you have the IRL experience of the brakes going out, because turning that valve on makes the pressure drop. As far as the SLRFL valve is concerned, it saw control voltage, it opened, it did its job. It can't tell a voltage put there by the ECU from a voltage that got there some other way.

 

It's ok to do wild-arse guessing, but it's important not to lose sight of your evidence, including which exact code you have. Many of the possible stories you are spinning here ("internal short (showing less resistance to the ECU than the coil should have)", "overheating damage to the coil"), would in fact not look to the ECU like a short to +12. A shorted coil would look like an excess current draw (INF 29), while a coil overheated and burned out would look open-circuited (INF 26 and/or 27). The solenoid coil cannot make +12 volts out of the aether. To have INF 28, that +12 must be getting applied to the circuit from somewhere.

The actuator is included in the troubleshooting tree for the simple reason that the electrical paths between its harness connector and the solenoids are inside it, so if you're looking for a circuit bridged to +12 somewhere between the ECU, wire harness, and actuator (inclusive), you don't get to prematurely rule any one of those out.

Priuschat member Dxta recently did some teardown of an actuator if you might be curious what it looks like inside.

For some reason, people often get funny about systems with ECUs in them, and like to jump right to suspecting the ECU for any issue. Generally people with more computing experience make that mistake less, but it still seems to be a temptation. In a car where things vibrate and corrode, it is still very common for old-fashioned electrical faults to develop for old-fashioned reasons. It is not that firmware never ever has bugs or that silicon never ever fails, but those tend to be the coin-lands-on-edge cases. In general, when an ECU that has been running in a million cars for a decade tells you it saw an unexpected 12 volts on a circuit, the top-of-list, Occam's-razor explanation is that it saw an unexpected 12 volts on that circuit.

In Gen 3, for example, there's this amusing (unless you have to pay for it) failure where the hazards come on and flash at a road-speed-dependent rate. It results from a short forming between two adjacent pins of the actuator connector (it's Gen 3, so the ECU is in there too). So far, no one has disassembled one of those actuators enough to find what exactly shorts, Maybe it's corrosion, or a tin whisker, or maybe even something got fused in silicon, in an FPGA or something. It's unlikely to be anything just in firmware, because the connection in question isn't even wired as an output from the ECU, it's an input.