I first noticed the frequent running of the accumulator motor in late 2012. It failed in late 2015. I would periodically, randomly measure the cycle times whenever I thought of it and make note. The length of time the motor was actually running changed very little over the years. Always in the neighborhood of 1-2 seconds. It was the intervals between pulses that changed and they were always perfectly consistent during each individual test. Typically, the times were taken with the engine at idle, in Park, and foot off the pedal, with no thought given to how much braking had taken place beforehand.

For those unfamiliar, this is a phenomenon that has been written about under many descriptions. Buzzing behind dash, vibration in brake pedal, etc. The sound can be heard from the drivers seat, felt in the pedal, and easily seen in the engine compartment by looking at the accumulator cylinder in the (pic) . If you're skeptical whether the drivers seat sound and/or pedal "click" sensation are connected to the accumulator cylinder, just time the sound and/or pedal sensation cycles and then go time the accumulator sound/vibration cycle (easily observed) in the engine compartment.

Fwiw, I charted the continuous cycle times (in seconds) that I had documentation for over the last three years up until failure.



If anyone has the cycle symptoms and decides to let the unit go to failure, which apparently can be many years, it's important to realize that if the system fails and even if you don't get one last good braking action from one last pedal push (which has been hypothesized), there will be brakes (along with a continuous warning beep and Christmas lights on the dash). You just have to be prepared to push them hard (they will be down near the floor).

How much braking power? Well, it literally depends quite a bit on leg strength. I have high relative leg strength but I'm a light weight, so as far as single, isometric push strength (male), probably about average, if even that. Obviously, anyone who is compromised for any reason in this regard would want to take that into consideration.

For me, I would estimate maximum braking power to be roughly the same as a somewhat quick deceleration for a yellow light, but not much more. Certainly not as much as one of those brake assist moments most people are familiar with. I never quite fully experimented with an all out push so I can only estimate.

The booster failed above 11,000 feet, with a difficult off-pavement descent yet to come. It turned out to be very fortunate timing. I soon got comfortable with the braking and came to realize that the braking action was in many ways superior to boosted brakes in those conditions (wish there was a way to turn off boost in low range). Nice, slow, smooth application of braking power with none of the abrupt stops that compress the front end that sometimes happen when a bump forces unintended pedal push. By far the worst part was having to endure the continuous beep, but I became comfortable enough with the braking action that I didn't panic when back in civilization and decide to pay to get it fixed 600 miles from home for no doubt premium pricing. Got home and bought a new unit from Boch Toyota South, thanks @eng208 . Thanks @blackworks for trying to help with the beep even though I decided against the attempt to disconnect. I had become so comfortable with the braking, I decided I'd better leave alone my incentive-to-fix, for my own good.

Drove the 600 miles home with earplugs which is a story in itself. I had nothing to do but enjoy the scenery and pay constant, close attention to my driving (in higher than normal gearing). Presumably, few have ever had reason to drive in such a manner but it was novel, a little surreal, and the time flew by in a way it seldom, if ever, has before. Would have never guessed.

This is great. Replied so I can find it again later!

I'll have to see how long mine goes next time I drive.

@JB. good you didnt panic and as a result learnt something from the issue,
mine followed a similar pattern..started notcing the pulsating brake pedals, and abt 20months after {became aware of the pulsing, so the situation might hv existed longer than that), it quit on me....
gave the metal canister some love taps and the unit worked again for 2weeks before finally giving up the ghost.......about 220 miles from home, managed to drive it home.....just drove carefully and didnt overtake much

drove my truck for about 40 to 50days with the poor brakes n constant beeping [bad idea}...just had to give plenty room from the vehicle in front and developed faster reflexes.....a quick 2 to 4 pumps of the brakes usually was enough to stop the car, albeit with a longer braking braking distance.
finally got a used unit shipped from a forum member and installed it...

had the aha moment when the used unit didnt work, the mechanic was stumped and said the used unit was defective, i did some thinking and decided to check the fuse that powers the booster, on doing that discovered that the 40amp fuse was bad...........guess the fuse shorted out from trying to actuate the motor assy in the booster, which was stuck....n thus resistance built up and it shorted the fuse....swapped out the 40amp fuse with another in the car,, and the booster CAME TO LIFE {WAS Ecstatic, hearing the good ol pulsing of the mc assy).

Biggest thing I learned is that I really like unboosted brakes on steep, rocky downhill trails.

Wow. Congratulations on not taking a hammer to the instrument panel.

Mine recently went out as well on my 2003 sr5. I was shocked to see how much it was going to cost to replace it. I found a great deal on the whole assembly off of a Lexus GS 300 and only used the booster motor and all is good. I actually joined this site trying to contact a member with a unit for sale

I have been having issues with my '02 4Runnerfor the last couple months--first the squawking noise, now the lights and alarm. So far the brakes haven't failed but I am getting concerned after reading people's posts. I don't think I have the leg power you guys are saying is required if it fails.

I took it to Toyota a few weeks ago and they quoted $3500 to replace it. I am trying to find a used one but with no luck. This car has been a POS since I got it. Unfortunately, getting a new one is not possible right now. Any suggestions would be appreciated.

Your 02 is a 3rd gen 4runner (not a 4th gen) so you'll get more help in that section.

3rd gen T4Rs - Toyota 4Runner Forum - Largest 4Runner Forum

Good luck.

I don't understand the chart. Is that pedal pushes between cycles? Mine runs for about 2 seconds, and while I haven't counted i would guess it's every 8 pedal pushes. Just wondering if I'm near the end of life. I have only owned my 03 for a month so I don't have anything to compare it too. About 100k miles on the clock.

The booster on my 2004 was replaced sometime before I bought it last year. According to the vehicle service history accessed through Toyota Owners, the previous owner was quoted $5300 by the dealer to R&R the unit.

The part itself through most dealers appears to be around $1400... Ouch

sounds very good, mined crapped out...the used unit i installed comes on every 2nd pedal push!...doesnt look good...thinking i may need to bleed the system more?...brakes are firm n stop the car nicely

No, the numbers in the chart represent the number of seconds between complete cycles of the accumulator motor turning on/off for 1-2 seconds....with an interval in between.....and then on/off again. So......motor comes on and then goes off 1-2 seconds later. When it turns off, the stopwatch would be started. Stopwatch lap button would be hit again at the next point in time that the motor turned off again. A 10 second cycle time would mean there was only about 6-8 seconds of quiet space when the motor wasn't running.

There was never any variance during the individual timing events. A 10 second interval would remain a 10 second interval at that point in time no matter how many times I hit the stopwatch interval button....10...20...30....40...50...

All or nearly all of them were taken with a warm engine idling in park and no foot on the brake pedal, though I did verify a few times that the pulses felt in the pedal were because of the accumulator motor turning off/on and that they matched what I was hearing through the dash.

Sorry JB, I'm not getting something here. You mean the pump runs every 10 to 20 seconds no matter what? As long as the truck is running this is happening? Even if you aren't touching the breaks...

So what am I hearing / feeling? I don't turn the radio on much so it is quiet in my unit most of the time. I can clearly hear the buzz and feel it in the break pedal. It certainly isn't happening several times a minute. Like I said, it is around every 8 break pedal pushes. Just my humble opinion, but if anything to do with breaking is running with any regularity without pushing the pedal you have a leak.

What are you looking for / hearing when you are timing these events??

Thanks for your input.
James

Yes.

It was cycling continuously regardless of the amount of braking being done (which I never payed attention to).

Any time I sat in park and let it idle (foot off brake) I could measure the cycle times.

I don't know what accounts for the inconsistency of the deterioration curve in the graph -- possibly due, at least in part, to my not accounting for the braking environment just prior to putting the truck in Park and recording the times, but the cycle lengths were always consistently the same at the time of measurement and I always took at least 4 or 5 measurements. If the shorter numbers (like the 13s in early 2013) were because of hard braking just prior to putting the truck in Park, you'd expect a healthy system to replenish and then stop. That wasn't the case.

Yes, I know I had a leak (presumably). This thread is meant to offer a possible suggestion as to how long the system might be expected to last under those circumstances. As to what I was measuring, it's covered in the first post.

Thanks JB. That makes sense now.

Mine runs every other pedal push, usually. But it depends on how hard I press the pedal. If it's just enough to turn on the brake lights, then I could do more many before it comes on. It's usually every 2 or every 3 for normal braking. If I pump the brakes a bit without fully releasing the pedal it can also come on.