Hydroboost Brake Booster Accumulator run/cycle history to failure

[hankd]

every second time i hit the brakes i hear this very quiet sound for two seconds that reminds me of a robotic stepping motor. i've only purchased my 05 runner a few days ago and was wondering if this was normal. brakes have been working normally. is this what the accumulator sounds like?

 

[AuSeeker]

every second time i hit the brakes i hear this very quiet sound for two seconds that reminds me of a robotic stepping motor. i've only purchased my 05 runner a few days ago and was wondering if this was normal. brakes have been working normally. is this what the accumulator sounds like?

It's normal, the power brakes uses a small electric motor to buildup hydraulic pressure to assist the master cylinder in braking, older style power brakes used a vaccum booster to assist the master cylinder.

You should also hear the small motor run when you first turn on the ignition after the vehicle has been off for several hours or over night, and then every second or third time you depress the brake pedal.

 

[earthrealm]

mine is on its way out again, it has a leak, and i hear the vacuum whoosing sound, when i depress the brake pedal, thus the motor is running more frequently, as because of the leak the motor struggles to pressurize the susytem sufficiently, it has failed 2 times in a serious stop n go traffic gridlock, pedal went to the floor, and all the alarms came on. parked on the kerb with the engine running and after about 5mins, the pump was able to repressurize the system.
came across a thread once that described how to fix the leak, but cannot locate it again, wish i ould see it, so i give it a shot.
meanwhile i am in the market for a used Master cylinder abs assy

 

[missionman]

I've been reading this whole thread and wanted to add my experience and what toyota told me vs what others say in this site on other threads.
I have a 2003 4runner v6. My accumulator pump was screeching and on its way out, however brakes worked fine and the pump barely ever ran. rarely did it run. Brakes worked great.
So I replaced the accumulation pump and accumulator (called a brake booster overhaul). Everything was pretty easy. I used toyota techstream to bleed the brakes numerous times on all 4 lines in a sequence.

Now here's where conflicting info comes.

I have always understood you must bleed the air from the brake booster itself by using the ABS program on techstream. problem is my version doesnt have that option so you must drive on dirt and slam the brakes on several times to release the air, then bleed the brakes immediately after. Some do this whole process numerous times and find air each time coming out.
Toyota told me I don't need to bleed the brake booster.

Now with my new pump motor and accumulator my pump runs every 3 or 4 stops.

Why would years of my old pump never hardly run except when I'd start the car and never run on any stop yet brakes worked great? Now they run seemingly a lot.
Toyota said this is normal but other toyota mechanics completely disagree.

The brake booster replacement suggests a sequence ran by techstream before first use. and then on first run you count the seconds on priming the pump for the first time. I did that and got similar results most get. all went well. it ran like 40 seconds or less. then ran again and it was like 13 seconds. then ran again and it was like 5 seconds. then youre done.

Does anyone know why my brake booster runs every 3rd or 4th stop and it never ever did this for 15 years!

 

[SocalSam]

My replacement booster assembly (from a 2004 Lexus GS300) runs 5-10 seconds on key on in the morning. After that, it does run from time to time but not enough to notice or bother me.

As far as bleeding goes, booster assembly has a master cylinder inside so it needs to be bled by having an assistant or tool box hold the pedal down while you crack the feed lines one by one. Then trigger ABS/TCS events and bleed again if needed. My brakes still feel great a year later.

 

[m85476585]

I learned something about the brake booster today. I had to bleed the rear brakes after some brake work. Just a little air got in. Before bleeding the pump would come on every stop. After bleeding it comes on every other stop, same as before. So the pump running more frequently could just indicate air in the system somewhere, as opposed to a failing pump/accumulator.