How often does the brake booster pump cycle?

Generally cycling when the car door is opened and at least once while braking is normal. Failing usually means pump operations frequently. Mine was every 15 seconds. While driving you can't really hear it in the cabin. Many hundreds of cars per Prius mechanic have seen the Brake Pump Accumulator fail and usually the Master Cylinder/ABS/Traction Control/Regen/Brake ECU (all one component) replaced as well. $2250 at my dealer.

As it stands now, only the 2010 Gen3 Prius and the 2012-2017 Prius v's are gen3s not covered by the existing special extended warranty by Toyota. This is an extended warranty with extended time unlimited miles. The 2010's time warranty expired in Dec 2019. The Prius v system is a different part number (maybe because of its unique Pitch and Bounce control for a better ride), and has never been covered, even though they fail as well.

It is important to realize the Prius brakes are essentially brake by wire almost 100% of the time. Regen during deceleration or downhill is the only braking at speed if the pedal is not aggressively pressed. Even at lower than 7 mph or when aggressively braked, the system depends on the brake booster pump and hydraulic accumulator to provide the hydraulic pressure. The car has a device that simulates brake feel on the pedal. All this to ensure high fuel economy by using regen as much as possible. The brake pads often last over 200k because they are not used much.

By the way, if it fails 100% there are no power brakes at all and you are hoping two normally open solenoids in the master cylinder // component does fail open as designed allowing mechanical brake hydraulics to have a chance to work. It is reported wrecks occur as drivers expect the good braking and regen to work and it does not. Sometimes because the failsafe does not work or sometimes because they don't apply unusually high brake force to the pedal. So they run into someone or something.

 

In answer to the "how often does it cycle" question, the answer is "depends on a lot of things." From a longer post elsewhere:

As rjparker said, it's not super easy to hear in the cabin while driving, depending on the amount of surrounding noise. Then sometimes you can. So it's even easy to get sort of obsessive about it just because of a change in how often you have noticed it running, which then makes you listen harder, and you notice it even more.

Sometimes a part will wear out and need to be replaced, and when/if that happens, you'll be getting codes and alarms or really, obviously frequent continual running of the pump. If that isn't happening, I would try not to get worked up about it.

 

You really can't count on that. The accumulator that gets pumped up to hold the pressurized fluid isn't all that big, about the size of your fist. The fluid can't even fill that much volume; there's the nitrogen pressure charge occupying part of it. If you watch your reservoir level from fully zeroed down while the pump runs first thing, it only makes maybe a cm difference in the level.

Every time you apply the brakes some fluid is used. It comes out of the accumulator, goes out toward the brake cylinders. When you release the brakes, the pistons retract and the fluid returns to the unpressurized reservoir. The harder you brake, the more fluid gets shoved out to the cylinders (really! under braking-level forces, metal calipers are elastic and stretch, you can see it with a dial indicator and it ain't even subtle), and returned to the reservoir when you release. Any slight wobble in the rotors will increase the amount of fluid used with every brake application (it tends to push the pistons further back when released, so more fluid returns to reservoir, and more is needed out of the accumulator for the next application).

The pump runs every time the pressure in the accumulator drops below a setpoint, and pumps it back up again. It'll typically be "every so many" uses of the brake, where "so many" is a smallish number, but depends on a lot. It probably runs more than you notice when you're not paying attention to it.

It certainly isn't going to run once at the start of your drive and be done, unless you have cut a hole in the floor pan and you're stopping with your feet.

 

As I suggested, the Brake Pump/Accumulator will initially run when you open the drivers door and most likely each time you press the brake. I recently had my brake accumulator and master cylinder replaced and can verify the before and after. My system was cycling continuously, taking about 10 seconds to satisfy then pausing 15 seconds before running again.

The nitrogen charge in the accumulator is separated from the brake fluid by a diaphragm of sorts (a metal pleated bellows). Just like a well storage tank, the compressed nitrogen provides immediate hydraulic pressure when you need it. The prime failure is the accumulator "diaphragm" leaking between the nitrogen and hydraulic fluid.

There is a pump runtime timer before the abs and brake alarms come on. One reference said five minutes. Meanwhile a new brake pump accumulator may take five seconds to repressurize the system. That nitrogen is also introduced into the brake/hydraulic fluid which also forces the pump to run longer as it has to pressurize the entrapped nitrogen as well. It is possible to get a day or two of close to normal pump operation by bleeding the brakes completely, ideally following the Toyota Techstream brake flushing logic which will open certain pathways at certain times. I proved that on my car before replacing the pump accumulator and master cylinder/ecu.

If your wife is not aware of the potential for much harder brakes with longer stop times this could be a dangerous situation for her, her passengers and strangers on the road she may meet. I would treat it seriously.

 

There is another failure mode that can also lead to excessive pump cycling and has nothing to do with the accumulator bellows. If there is any small internal leak in the hydraulic system, say an incompletely seating valve, that allows the pressurized fluid to slowly escape the accumulator and return to the reservoir, then that will also lead to more pump cycling, even though the nitrogen is staying right where it belongs. Most of the valves that could be culprits in that scenario are in the actuator assembly rather than the accumulator assembly (but not all of them).

I don't know that we have enough information to say which failure is more common, or the "prime" one. For the record, the trouble codes indicating "small internal leak" refer to the hydraulic kind. There was a recall about accumulators with bellows prone to leaking nitrogen, but that applied to a run of accumulators where the metal bellows were made slightly undersize, so they bounced around inside the can while driving, and developed cracks. Those accumulators could be recognized by the orientation of the label.

In my experience, it's not usually each time the brakes are used, but every smallish number of times. Harder braking will deplete the pressure faster, as will longer brake travel (related to disc runout, for example).

 

Agreed. After the 2010 "recall" Toyota started replacing both the accumulator and master cylinder. In my case we did both as well although a flush of the system would "solve" the problem for a day or more indicating a hydraulic leak was not the likely cause. In a sense the real issue is Toyota has left off the Prius v when even the dealers tell me plenty have failed.

 

I'm rite in the middle of all of this.......... one thing I have noticed is that when moving very slow and my foot on the brake I can feel magnetic pulses in the entire vehicle.... feels like a PMA pulse but could be the antilock.
Pump runs excessively.

 

It is the pump operating changing the pressure. Feels something like warped rotors. You won’t feel it when its holding pressure better.

 

I need to get this changed out....... Going to need the car for a couple of road trips......
Just thought I'd document the pulsing....... I knew that wasn't normal.
Still tempted to do just the pump.....just to see what happens.... might do that then still do the master after testing it.....not putting it back together just bleed and see if it runs. Would be silly not to do both since I have them. I actually have 3 pumps..... and I have 2 complete egr systems also.

 

P

 

I'll be glad when I'm that far........

 

Supposedly 53,000 and a total of $215.00......... and off a Lexus

 

Ya...it never did it before.......

 

Before doing anything, It is FREE REPLACEMENT by Toyota https://static.nhtsa.gov/odi/tsbs/2019/MC-10164922-9999.pdf . 31 August 2021/unlimited mileage.

 

Not for the v.

 

H᠎i, welcome to PriusChat, where a lot of people come looking for accurate information about their Prius cars.

The 'campaign ZJB' document linked above applies only to 3rd generation Prius liftback and Prius Plug-in (PHV) cars, not to the v or the c. The end dates vary by model year:



So it's already over for 2010s (unless anybody's got one that hasn't rolled 150k by now).

For 2011s through 15s, it is still in force through August 2021 and then to your tenth year or 150k miles, whichever comes first.

It is a replacement conditioned on specific failures ("certain internal malfunctions of the brake booster and brake booster pump assemblies"). It is good to remember that in the 3rd generation Prius, those are two different assemblies:



There is a list of trouble codes that may mean the covered failure has occurred. They are codes that can also be set for other reasons. Sometimes a few simple steps of diagnosis will reveal that something else has caused the codes you have, and that replacement of one or even both these assemblies would not be the necessary solution.

 

...

 

This is an update: The 2012 Prius V is now included in a Customer Support Program for this issue. You have to call and speak with Toyota Customer Experience Center, provide your VIN and talk about the issue. I received a letter about this, I knew about my C1391 code (having bled the brakes and reset the code with tech stream; I needed drive the car while I figured out how I wanted to do the repair). When the code and warning lights came back again a couple months later, I had already received my letter, so I was ready to take it into the dealer. The dealer fixed the car with no cost to me and included a rental. The program does suggest that if you fixed the car (at a dealer) and paid for it, you can get reimbursed from Toyota. As other forum members have said. The electric hum of the faulty booster will be heard as you open/unlock the car, while you drive, and after you shut off the car. Once repaired - I barely hear it briefly when entering car - and don't hear it while driving or after car turned off. I want to thank all forum users for input - I learned enough to save me a ton of money on this costly repair.