A Brake Question for Priuses

(I would check your 12 volt battery, when mine was weak, I got brake warning lights)

If you press hard enough, the brakes work without electricity.

They do have a great many alternate 'paths' to engage.
Figure 1-11 from HYBRID COMMERCIAL VEHICLE (HCV) DELIVERABLE D2300.1 TECHNOLOGY EVALUATION REPORT FOR ELECTRICAL ACTUATED MECHANICAL BRAKES AND ELECTRICALLY POWERED STEERING SERVO | Semantic Scholar

 

In normal operation, the fluid you push in the master cylinder is not going to the wheels: SMC1 and SMC2 are closed. The fluid is going into a stroke simulator (SCSS is open), just a cylinder under the hood with a piston and springs, made to feel the same as if you were braking.

Pressure sensors PMC1 and PMC2 measure the fluid pressure that you are generating in the stroke simulator, and that is the input telling the brake ECU how much slowing you want.

The brake ECU asks the HV control ECU how much slowing can be done through the powertrain. The HV control ECU replies with an answer, the brake ECU subtracts that from the amount of slowing you wanted, and applies hydraulic fluid through valves (those SLAxx and SLRxx valves) to do the rest.

If the brake ECU loses power, or loses confidence in its own sanity, it will drop power to all the valves. SCSS will drop to its normally closed position, so your foot pressure is no longer wasted on the stroke simulator, and SMC1 and SMC2 drop to their normally open positions, so the fluid pushed by your foot acts directly on the two front calipers, and only there. (In this fail-safe mode, the rear brakes are out of the picture.) You will have to press super hard, as there is no boost action in the Gen 2 master cylinder.

Many of the details I've just given are Gen 2-specific, and differ in other Prius generations.

 

the lights can mean a lot of things. you need to read the trouble codes with a prius appropriate scanner

 

By federal passenger car safety regulations, they must still work even when all brake booster power has failed. If they don't work, it is a defect for which the car should be recalled.

But as Chapman pointed out, when power is lost and fail-safes come into play, you must press the pedal very hard, just like back in the old days of unboosted manual brakes. Put all your body weight or more (braced against the seat) into the pedal. Federal regulations specify a maximum (but long) stopping distance when the applied pedal force is 112.4 pounds (metric 500 newtons). Unless you have very strong legs -- it is best to use both feet -- you probably won't have a short stopping distance.

 

By contrast, in Gen 1 and Gen 3, the master cylinder includes a boost chamber, and it will power-assist your fail-safe braking, so you don't have to push as hard, for as long as the accumulator pressure holds up. That can normally give you 20 or 30 strokes, which is pretty nice (the vacuum boosters on older cars would only give you 2 or 3, if the engine stalled and wasn't pulling vacuum). After you use up the pressure, you're back to having to press super hard.

Gen 4 also has a boost chamber in the master cylinder, but its boost servo regulator is moved away into the actuator, and I've looked at that diagram till I'm crosseyed and I'm still not sure if it can give you any boost during fail-safe.

 

Great explanation, the more I read about this car the cooler it is to me.