Been searching and reading. There is a lot of information. It seems like the most common failure with the brake master/booster assembly is the motor wearing out and failing in various ways. And it seems to give hints that there are problems which is usually some loud sounds from the motor.

I have an '05 SR5 V6. It has 195K miles on it. I've owned it for 2 weeks. Aside from the usual maintenance issues, the big concern for me is that I can hear and feel the brake boost pump come on every 10 seconds, lasting for about a second. I can feel a slight clunk in the brake pedal and steering wheel every time it comes on. It does whenever the key is in the ON position, regardless if the vehicle running, sitting at idle, running down the road, brake pedal depressed or not. My sense is that the accumulator is not holding pressure or a pressure sensor is not sending the right signal. The motor isn't noisy or loud, just cycling on/off a lot more than I think it should.

I'm not inclined to just start replacing parts whenever something isn't working. I'd really like to figure out how to diagnose if it's the motor, accumulator, ABS valving/solenoid assembly or the master cylinder. I also don't like throwing money away on complete assemblies when it is often something simple. I have a friend with Techstream that thinks he can run a diagnostic but I don't know what I can expect to get out of it. I have not come across this specific failure mode in any of the threads that I've encountered through extensive searching.

Suggestions?

not sure if this will help, but i replaced the abs brake booster in my 4runner in 2013 w/111k miles. for a while i kept hearing a regular squeaky noise from that area of the truck while driving. never stopped. toyota diagnosed a failing booster, quoted about $2500 for the job.

ended up buying a used unit and having my independent shop replace it. cost $150 for the unit and $250 for labor. at 200k now and haven't noticed an issue.

I've been keeping my eye on the U-Pull-It yard inventory to see if the correct year shows up. I should figure out what the interchange is on the part I need to see if any other years/models come up that have the same part.

I would go to the dealer first to get the answer. maybe do a full inspection for $100. talk to parts as well. i start at the top before i go to secondary sources like junk yards.

There's some troubleshooting you may be able to do.

Consider bleeding the brakes using the spiffy easy procedure in the shop manual:
https://www.customtacos.com/tech.old...a/04200210.pdf

You'll need a helper to do the individual calipers. Start w/ the pedal pushing procedure to bleed the master cylinder, then move to the calipers. I noticed similar extended booster run time after replacing my rear calipers when I put the key in RUN position to shift it into Neutral...it went back to normal after they were bled along with the fronts.

edit: get plenty of new brake fluid and push it all the way out to each caliper, bleeding each one until clean fluid is leaving the bleeder.

As much training as the dealer techs get, you'd think they would be the experts. They have the tools at least. But I don't trust the dealerships very much. Maybe I'm just cynical but I have always felt like they are out to make as much money off me as they possibly can. And I've taken vehicles in for warranty work and had them come out worse off than when they went in.

I do have some guys I trust at the parts counter but they don't know squat about troubleshooting/fixing them.

This still may be the route I go after I've exhausted all my other options, including the buddy with Techstream.

Thanks for the suggestion. I will check it out.

i'm not suggesting doing the work at the dealership. definitely the highest cost route. not sure how long you've owned yours, but i like to pay for an hours labor to get a full report on my vehicle and get info for specifically known issues, then go from there.

I've only owned it for a few weeks. It feels like I am hemorrhaging money. Now I know why it seemed like a good deal. Funny thing is that the PO probably didn't even know there was a problem. He seemed pretty oblivious to all things car related. I just replaced the spark plugs and discovered they are the original ones (3 NGK and 3 Denso) with 195K miles on them.

if you didn't do a pre-purchase inspection, you should do a post-purchase inspection at the dealer. worth an hours labor charge to get a clearer picture of the vehicle. you may decide to resell and start over. you could very easily get into thousands of dollars in repairs in an older vehicle.

"I have an '05 SR5 V6. It has 195K miles on it. I've owned it for 2 weeks. Aside from the usual maintenance issues, the big concern for me is that I can hear and feel the brake boost pump come on every 10 seconds, lasting for about a second."

* I would inspect all the brake lines from the booster to the brake calipers looking for any signs of a leak.
* Consider having a brake fluid exchange done on it at the dealer so that they do it per the Factory Service Manual. There could be air in the lines or in the hydraulic assembly itself.

My booster assembly pump started running every 10 seconds or so followed by a sqwelch like noise. I replaced the accumulator and the problem went away for 2 months, then re appeared off roading one day. I unscrewed the accumulator let some brake fluid out and re tightened it. that was two weeks ago and no issues since, so I`m not sure whats happening. My guess was air since I never bled the lines when I first replaced the accumulator.

Hope this helps a bit.

I have a little more information but no clear solution yet. My friend with Techstream hooked up and ran some diagnostics.

There is an "Accumulator Sensor" listed as one of the parameters which I can only assume is a pressure sensor. It outputs voltage. I could see the voltage fluctuating between 3.3V and 3.8V. When the voltage would drop to 3.3, the pump/motor kicks on. You can see the voltage increase to about 3.8 in 1-2 seconds and then the motor shuts off. Then you watch the the voltage drop back down over a period of about 10 seconds and the cycle repeats.

With the key on but the engine off, it's quiet enough that you can almost hear the sound of something leaking. All indications are that the accumulator is not holding pressure. Now if I could just locate a boost motor and accumulator for a not outrageous price I'd be happy. A used assembly on Ebay is $400+ which is beyond my threshold of pain for buying parts that I'm certain will fix it.

The accumulator appears to be replaceable separately, and without removing the booster assembly. However, I would still bleed the brake system first. Your truck has a lot of miles on it, and who knows how old the fluid is & if/when the calipers have been bled. Some people do brake jobs just by compressing the pistons and don't crack open the bleeders at all.

See this post where a guy responded that his pump was cycling as you described, and corrected it with a bleed:
Brake Accumulator change procedure?

So, I know I'm attempting to bring back an old thread but here it goes.

I'm having similar issues with my booster pump/accumulator. Mine seems to cycle every few presses of the pedal. It's an 03' V8 btw. It always runs for 15 or so seconds once the key is in the on position. The pedal has definitely seemed soft for a little while now and braking has been a bit sub par but manageable.

I just replaced the front end with new calipers/rotors/pads and went through all the bleed procedures I could find short of the solenoid one using a scan tool. I do get straight fluid on all 4 wheels after numerous bleed attempts just to make sure. Here's the strange thing and the reason I decided to post on this thread, I found the link on whole to bleed the master cylinder in the link earlier and decided to give it a shot. It states: turn key to ON, wait for pump to stop running, turn key to off and pump pedal 20+ times to release pressure from accumulator. Pedal stroke will lengthen and become easier once pressure is released. Repeat process 5+ times till bleed. Here's the kicker: my system seems the opposite of what's described??? Pedal stroke is soft for 20+ strokes THEN becomes stiff? Repeating the above process yields the same result each time. I've gone through it 10+ times now. Seems to me like the accumulator isn't holding pressure but I'm not well versed in this Toyota booster assembly. Any thoughts are greatly appreciated before I consider taking this ol girl to the dealership over the issue. Thank you all in advance!

Have replaced the motor and accumulator on 2 of our 4runners....2003, 2008 and 2009. Couple of good youtubes on pulling master cylinder complete assembly out....and then putting in new motor and accumulator......not a hard job for good DIY person, I chose to pay $200 to local trusted mechanic and bot the 2 needed OEM parts new, for both. Paid about $900 total for them....so total out of pocket $1100. Much better than dealer quote of almost $4000....said they only replace entire master cylinder assembly. The motor failing and accumulator failure are by far the main failures that happen to master cylinder on 4th gen runners.

I'm having similar issues with my booster pump/accumulator. Mine seems to cycle every few presses of the pedal. It's an 03' V8 btw. It always runs for 15 or so seconds once the key is in the on position. The pedal has definitely seemed soft for a little while now and braking has been a bit sub par but manageable.

I just replaced the front end with new calipers/rotors/pads and went through all the bleed procedures I could find short of the solenoid one using a scan tool. I do get straight fluid on all 4 wheels after numerous bleed attempts just to make sure. Here's the strange thing and the reason I decided to post on this thread, I found the link on whole to bleed the master cylinder in the link earlier and decided to give it a shot. It states: turn key to ON, wait for pump to stop running, turn key to off and pump pedal 20+ times to release pressure from accumulator. Pedal stroke will lengthen and become easier once pressure is released. Repeat process 5+ times till bleed. Here's the kicker: my system seems the opposite of what's described??? Pedal stroke is soft for 20+ strokes THEN becomes stiff? Repeating the above process yields the same result each time. I've gone through it 10+ times now. Seems to me like the accumulator isn't holding pressure but I'm not well versed in this Toyota booster assembly. Any thoughts are greatly appreciated before I consider taking this ol girl to the dealership over the issue. Thank you all in advance![/QUOTE]