Within the past month in Ohio, the weather has gone for weeks of below freezing to below 0 fahrenheit. Currently, it is 45 with the last few days being 50-60. Some symptoms about my 350K 2007 Prius: 1. Loud beep at startup approx 10seconds 2. BRAKE light on dash far left light near door 3. Intermittently no brake pressure...ie soft brakes 4. NO CODES via Autel Toyota OBD (brake related) Watched several videos about possible causes, brake actuator being the primarily mentioned issue. However, there is no noise from the brakes (whirring sound) but sometimes there are a few clicks from that area where the BA is. Car seems to run fine otherwise. My feeling is that the sudden-change in warming temperature caused some expansion on the BA and maybe the issue might be the lines going into the BA are looser or something to do with brake not being distributed evenly. I checked the brake-fluid and its OK. I understand that 350K is a lotta miles and its probably the BA but I'd like to exhaust other possibilities before undertaking this job. As a sidenote, I'm semi-experienced with mechanical-work but how involved in time & effort would it be to do a replacement, I'm more worried about the install not being 100% tight and still having this issue. At some point we all gotta let these things out to pasture so I'm kinda on the fence. Thanks for any advice.
You should hear the whirring sound even on a good actuator. First when you open the driver door, and every so often when applying the brakes. First thing to check in cold weather is the 12v. Get a free load test at most auto parts stores
thanks. I'm gonna put the 12v on the charger today then. just curious, how the 12V plays a role in the brake actuator so i asked GROK AI. the answer: does 12v battery in 2007 prius affect brakes Yes, a weak, low, or failing 12V battery in a 2007 Toyota Prius (second-generation model) can affect the brakes — but usually not by making them completely fail or lose all stopping power. Instead, it often triggers warning lights and related issues. The 2007 Prius uses an electro-hydraulic brake system (with a brake actuator) that relies heavily on the 12V auxiliary battery to power the electronics, computers, sensors, and the brake system's control modules. The high-voltage hybrid battery handles regenerative braking and propulsion, but the 12V battery is crucial for overall vehicle electronics, including brake-related systems.Common effects from a low or dying 12V battery include: Brake warning light, ABS light, and/or VSC (Vehicle Stability Control) light illuminating on the dash. Brake system entering a fault mode or fail-safe, which can cause a harder brake pedal feel, reduced assist, or messages like "Check braking system" or "Apply parking brake" prompts. In some cases, the brakes feel "spongy," only the rear brakes seem to work properly, or there's unusual behavior when pressing the pedal (e.g., during startup or while driving). These issues often appear when the 12V voltage drops too low (e.g., below ~10.5–11V), causing erratic sensor readings or module errors. Many Prius owners report that replacing a weak 12V battery clears these brake/ABS/VSC lights and resolves related symptoms, as low voltage causes "ghost" codes or temporary faults in the brake actuator system. It's a common first troubleshooting step before assuming a more expensive issue like a failed brake actuator (which is a known problem on Gen 2 Priuses but typically shows different symptoms, like constant pump cycling noises). If your 12V battery is old (they usually last 3–6 years), low on charge, or failing, it's worth testing/replacing it first — auto parts stores often do free load tests. If the lights persist after a good battery and reset (sometimes by driving or using a scan tool), then scan for codes or check brake fluid/sensors/actuator.If you're experiencing specific symptoms (e.g., lights on, hard pedal, no braking power), describe them for more targeted advice! Always prioritize safety — if brakes feel compromised, don't drive until checked.
We had a Civic Hybrid where the "unbelted front seat passenger" light would come on intermittently whenever it rained - with nobody in that seat. There was no water intrusion, it wasn't particularly cold when this happened, so I always assumed it was due to humidity, somehow. We also know that Prius cars that are located in humid areas are more likely to develop corrosion in the HV Pack computer than those sited elsewhere. Temperature changes of course can screw with things, as can water intrusion. I wonder if barometric pressure changes ever make cars glitch.
who knows? I still can't figure out how exactly the brakes work. I feel temperature did play a part in how the brakes are acting but we'll see.
Once the car goes to Ready your 12v battery does nothing but receive a charge if it needs it. The inverter/converter takes high voltage and converts it to 13.5-14.5vdc for all the vehicle's 12v loads. A soft pedal means your brake booster has quit - a dangerous situation. It may recover now and then but its history.
I have a feeling its my 12V battery charge. Resting it is at 11.5ish and turned on car engine it goes to 14V. I looked into why the 12V is necessary for the actuator and it is. I figured there was a capacitor in the front to power these pumps and internal electronics (like on the dashboard lights) but apparently there is none. could you kindly explain HOW the braking system works in simple terms? I understand there is an accumulator but even that doesn't have internalized power. I guess what I'm asking is ....does the initial start of the car 12v battery cause the actuator to work after rest OR is does the actuator only take a charge when it needs it??? from my reading, it seems they changed it in the models 2010 et al going forward. Thanks.
The Prius brake accumulator is just a pressure tank that gets pressurized by an air pump that runs off the 12v battery during the turn-on process. The pump runs periodically of the 12v bus (then powered by the traction battery) while you are driving. the air pressure in the accumulator is the main power source for the friction brakes and the simulated operation of the brake pedal so that the driver gets feedback from the brake system even though most of the braking is actually done via regeneration (Generating electrical energy from the drive system and charging the traction battery). Toyota engineers did a magnificent job designing our Prii. JeffD
That's pretty much the story, except that the pump is a fluid pump, not an air pump. The accumulator does contain some 'air', really just nitrogen, trapped (almost) forever behind a bellows. When the pump runs and pumps brake fluid into the accumulator, the nitrogen is further compressed, so the fluid is under enough pressure to work the brakes. When the pressure sensor reads the low-trigger for working pressure, the pump comes on and runs for a second or so until the pressure reaches the high trigger and the pump shuts off until needed again. The water pump and pressure tank in a house with well water work pretty much exactly the same way.
