Hello All,

I have this problem

YouTube Video

It only happens in the morning and and only happens once a day.

I really do not want to have to pull and swap the entire Master Cylinder if the solution is just the ABS Pump or one of the pressure sensors.

Has anyone solved this problem? Thanks in advance!

a few questions, first

1.) does the CEL light come on or any dash lights for that matter
2.) hows the brake pedal fill (does it go to the floor or is it firm and engaging high)

Just from a a little knowledge of pumps if a pump sucks in air it can't build pressure if its ran off of a pressure sensor switch it will continue to run until the pressure switch its activated and the pump becomes fully "primed". If you have a crack in a line or any leaks on your pressure side the same thing will happen.

from the sound of the intermitancy of the problem it seems like maybe a faulty pressure sensor. or some other circuit is telling the abs system that the pressures not being reached (i.e morning dew, condisation, high humidity, can cause short circuits) as well sometimes hot and cold can contribute to electrical problems like when things heat up they tend to expand so maybe there a stretched wire that once is heated expands to make the connections

just a few thoughts off the top of my head without really having any experience with your issue hope you get it worked out

Unfortunetly, I had to deal with a different problem that wound up pointing to the master cylinder assembly. After talking with TAS (Toyota's factory help line basically) the guy let me know that there is no way to diagnose problems internal to the unit and that diagnosing the master cylinder assembly is a matter of elimination.

Also unfortunetly, we do not sell individual components for that assembly. If you suspect the ABS modulator is the problem you have to replace the entire master cylinder assembly. Sorry to be the bearer of bad news.

No lights on the dash and the brakes feel normal. Just that chirping/squealing sound in the morning and once the engine is up to normal temp. it is gone and does not happen for the rest of the day.

Sounds like a belt, but I have to watch the video.

Something is wrong with the acumulator pump or resiviour. Not much you can do about it...

Actually, I believe Toyota does explain how to disassemble the whole master cylinder assembly, which is weird because you can only get the whole unit and there are no diagnostic steps you can use to test it.

On vehicles with integral anti-lock brake systems where the master cylinder is part of the hydraulic control assembly (Teves Mark 2 ABS, Bosch III ABS, Delco Powermaster 3 ABS, Bendix 10 and Jeep ABS), an electric pump with a nitrogen pressurized accumulator is used to provide power assist.

With these systems, power assist is provided by pressure stored in the accumulator. We're talking LOTS of pressure here, from 675 up to 2600 psi depending on the system and application. When the driver steps on the brake pedal and the pushrod moves forward, it opens a valve inside the master cylinder that allows stored pressure from the accumulator to enter a cavity behind the piston assembly. This pushes the piston forward and applies the brakes.

A pressure switch on the master cylinder monitors the stored pressure in the accumulator, and closes a switch to turn on the electric pump when pressure drops below a preset minimum. It then turns the pump off when pressure is back up to where it should be.

Problems with this type of power brake system will usually be due to a bad pump motor, a leaky accumulator, or internal problems in the master cylinder assembly. Because it's all part of the ABS system, electrical problems with the pump motor or pressure switch as well as low fluid level or low pressure will usually trigger a fault code and activate the ABS warning light. To find out what's wrong, you'll have to hook up a scan tool or use the appropriate diagnostic procedure to pull out the trouble codes. Accurate diagnosis here requires referral to the applicable shop manual.

The electric pump and accumulator can usually be replaced separately if there's a problem, but the master cylinder and hydraulic control unit are replaced as an assembly (which is very expensive!).

The most important thing to keep in mind about servicing these systems is to always depressurize the accumulator before working on any part of the brake system or opening up any plumbing. The pedal needs to be pumped 30 to 40 times with the engine off (or until an increase in pedal effort is clearly felt) to bleed off all the pressure from the accumulator.

If its not throwing codes its almost imposible to guess at. But with it only happening first in the morning. I feel extremely confident that theres something screwy in the electrical work behind the pressure sensor, pump motor, and anything else on that circuit. If it were my vehicle I typicaly follow the drive it till it breaks method if i can't narrow down the problem lol. But i would def. inspect all the electrical connections and wires on those circuits sometimes just adding electric grease to connectors can fix things its weird. best of luck.

Never mind, you said it sounded like it was squeaking/ chirping only on damp/ cold mornings so I thought it was a drive belt. Never mind on that though.

That def isn't normal, but it sounds like it coming from the inside of it.

Good look getting it fixed!

Hey AHA2988, do you have a source for individual components for the hydraboost system? As I said before, from the factory we can only get the entire assembly as one piece; MC, ABS Module, Tank, pump, ect. But I would be very interested if I could pick up individual compoments.

Also, unfortunetly my experience with the hydraboost systems we use in the 4Runners is that they will not always throw codes for pressure problems internal to the unit. For example a lot of the "Soggy Pedal" issues don't set a DTC, but are internal issues within the unit.

BUT, if you've managed to find some diagnostic tips on testing these systems please let me know, because TAS was useless in helping me diagnose the system. They're diagnostic steps were, "If you did everything else, then replace the hydraboost unit." Which quite honestly annoys and confuses me, because in the repair manual there is a very detailed procedure for rebuilding the units.

just doing a quick search on google i found this, i would imagine there are other sources but I've havn't needed to purchase anything

BRAKE MASTER CYLINDER. Fits: TOYOTA | Village Toyota Parts

I looked in the few manuals i have and they all say check connectors, fluid level, wiring harness's, and if none of those validate the problem to take it to a "qaulified tech" lol no offense just ironic because your proceedure is to just replace the whole thing. but it looks like you can buy a few of the componants pretty cheap the pump and the hydraulic control unit are the big kickers

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I was playing with mine yesterday just to see if i could get it to make ur noises and noticed my pump motor is running much more quiet than yours. when i turn my switch to the on position the pump motor kicks on you can kinda hear the pressure building as the motor sounds like its loading up then when the pressures reached i hear a somewhat of a clicking sound and while watching the actuator i noticed when it stops building pressure and the clicking noise is heard the actuator actually jumps up a little. Also if you follow the link in my previous post youll see somewhat how the actuator mounts it looks like its on a spring and theres a housing around the actuator that holds the actuator with rubber stops allowing the actuator to go up an down as its ingaged and disigaged (my assumption). But i noticed in your video it seemed like you got the motor to turn off when you pushed down on the actuator right at the end of the video. Perhaps theres something wrong with the spring holding the actuator up or something in the housing holding up the actuator from moving around the way it needs to. just tryin to rack my brain i mean if i had or have this problem in the future i'll def want to know the steps to take


I'm not exactly sure how to remove the actuator or tear into it at all really i do know though that when the vehicle is off as you pump the brake pedal it releases the pressure in the actuator. ( i had to do this to get the pump to come back on when i turned the ignition switch to on each time) so i would def recomend pumping that brake pedal atleast 50 times count it out to. if you decide to investigate anymore into it. you def. dont want to release 2500 psi uncontrolled lol

Well, I certainly am thankful to all that have weighed in on this one.

I am thinking that it is the pressure sensor that its not reading the pressure correctly there by making the pump run too long until it squeals.

Since the pressure sensor is 300+ by itself I have found a used master cylinder for 125 and will examine whether I can pull the pressure sensor and swap it out.

If I can avoid having to swap the entire unit I will.

I appreciate everyones help on this one and I will post again if I get a resolution from this plan of action.

Well, there has been a change in the problem.

So plenty of time has been eaten up trying to get a master cylinder / booster from car part dot com.

Plenty of the yards list the parts but then they are already gone. One place called me back AFTER I gave them my credit card to tell me that they priced it to low and were going to have to charge me more than double!!

In the mean time I found a solution.

At night, when the 4runner is done for the day, I would depressurize the system by pumping the brake 20-30 times with the engine off.

In the morning I would not have the problem.

This was done for 3-4 weeks.

Now the problem does not happen at all.

So was this just an air bubble in the booster? Or a pressure switch that just needed a break for a while? I do not know.

But if the problem is gone, it is gone. I am good with that.

So it's been another month. Is this still fixed? I'm having the same problem with mine...

Thanks,
Baron

any update???,,,,, i hv been hving this problem for some time now, n i think its getting worse or am getting more sensitive,,,,now with the car off n the ignition on, i feel the pumps come on for abt 8secs every 2mins,,,,,,,i dont think this is normal, with the car off it shud just pressurize the system once n quit, not the periodic cycling,,,,,, this maes me feel the pumps still cycle lie this when the truck is on/moving just that the engine noise masks the sound