Brake Booster & Brake Booster Pump Assembly Support

There is a failsafe in the brake by wire system that allows a pair of valves to fail open which results in straight hydraulic braking. However it is not power brakes at that point so the normally good ecu controlled brakes are reduced to a system with longer stopping distances. The odds are good failure will occur gradually, eg the weak brake booster pump will take increasingly longer to maintain standby pressure, eventually tripping a code prior to the manual failsafe mode. I would drive it and stop safely if the warning lights come on.

Background:
How it works is a long story but to give you a general idea, the pump/accumulator builds up pressure in advance which the master cylinder/brake ecu directs to each wheel. The pump assy has a hydraulic accumulator tank (essentially a pressure storage tank) which has compressible nitrogen pushing a diaphragm to presurize the brake fluid. Much like a bladder type well tank. The diaphragm often starts failing and leaks nitrogen into the brake fluid. When that happens, the pump runs for longer periods to maintain a preset pressure. However, if it runs long enough (perhaps five minutes) it will set a code. The system can still work even though its taking longer to build pressure. The nitrogen gas gets into the master cylinder assy and often causes it to fail. On a gen3 the pump/accumulator is a separate device from the master cylinder/brake ecu, but Toyota replaces both based on experience and the amount of labor needed to access either part.

Parts: 2013 Prius v wagon - this forum - std Prius will be different parts
MSRP and McGeorge Toyota Online Prices

1. 47070-12020 Pump Assy $913 MSRP, McGeorge $696, Used $164




2. 47050-47110 Master Cylinder Assy $1405 MSRP, McGeorge $1072



3. 44785-02060 Gasket, Brake Booster $5 MSRP, McGeorge $4
4. 00475-1BF03 Brake Fluid $22 MSRP

 

There's also a devilishly confusing point in Toyota's nomenclature: the "booster pump" is not the "booster", and the "booster pump" is the thing that's combined with the accumulator, and separate from the master cylinder.



The "booster" is not separate from the master cylinder, actuator, and ECU; in fact that upper assembly there combines the ECU, master cylinder, booster, and actuator, and is sometimes called all of those things.

The "booster" is a tiny chamber at the rear of the master cylinder piston, and a reaction disc/valve that goes with it. What the booster does is, all hydraulically, respond to your foot pressure on the piston by letting in some of the high pressure fluid from the accumulator, also behind the piston, so that it amplifies your push. This still happens, even if the actuator electronics are shut down, for as long as there is fluid pressure in the accumulator. That can be good for about 20 or 30 uses of the brake, so even in fail-safe mode, you are not yet without power-assisted brakes ... until you use up the available pressure. Then the pedal will become very stiff, and you'll have to stand on it hard to slow the car.

That booster can be compared to the vacuum booster in a conventional car, except those usually only have enough vacuum on tap for 2 or 3 uses of the brake in the absence of engine vacuum.

There is another kind of leakage failure possible for these brakes that has nothing to do with nitrogen leaking from the accumulator. There are about a dozen little solenoid valves inside the actuator, and if any one of those starts failing to seal correctly, brake fluid (not gas of any kind) may leak past it and return to the reservoir. This is another common reason for the accumulator failing to hold pressure and the pump having to frequently cycle.

One last thing about the booster and the 20 or 30 strokes of power assist you have under fail-safe conditions: if you ever are driving a Gen 2, don't assume it works the same way. That system was quite different, and does not have a booster. The Gen 2 system cannot give you power assist without its electrical supply. For that reason, Gen 2, and only Gen 2, has a big box of capacitors in back to shore up electrical power to the brakes. When that goes out, the Gen 2 fail safe is purely unassisted, and you have to stand on the pedal.

 

I have the same problem but need to drive the car on a 600 mile roundtrip. Am I crazy to think about doing this? I've never added any brake fluid to it and it's still on max. No drips under car. I used to drive a willis jeep with no brakes for fun.... but that you could downshift and had ground clearance for jumping curbs. What are the chances for a catastrophic brake failure?

 

It doesn't leak fluid. It leaks pressure from one area to another.
Mine ran a lot then went out and I barley made it to the dealership 3 miles away.
Almost 0 stopping power.
What are you experiencing?
From your post it seems as though u have no idea how the regenerative braking system works.
U better be careful. ... could get your self our somebody else killed.

 

If the car has 149,401 miles you are crazy because you would drive it through the warranty expiration for this part. Make sure your liability insurance is at least a million and be sure your emergency brake works. There is a failsafe mode but you lose all power and regen assist. Usually by surprise at the worse possible time.

The timing of the brake pump and your mileage are key parameters as long as you are ok with risk.