[werminghausen]

I have not had a car with such complicated brake booster/ABS than the 4th Gen.
My brakes quit 3 years after I bought the 4th Gen (2002 model).

When I bought the car I was highly suspect of these brakes (that I don't quite understand) . I am not good at instructions if I don't understand what I am doing.

I had a weird brake failure and I still don't know what was happening in detail.
I have to say before the incident I noticed a softer brake for some months.

Then the brake sign lit up in the dash all of a sudden but nothing else. The brake still worked.
Now I know that the brake dahs light is the sign for low brake fluid level.
I continued driving (Had not much choice)

Then 10 minutes later the whole dash was lit up together with an awful noise and soon after - no power brake. I had to stop and tow the car home. At home I saw that the brake fluid tank was empty.

I was searching for the leak and found one rear brake line rusted out and damaged. At least I found something....and I replaced the line. The system was tight again.
But still there was the alarm on: beeping and dash warning lights.

Then I replaced the hydr. pump/el. motor of the brake booster assembly.

The new pump helped to get rid of the dash lights, what a relief.

I was bleeding the brake 2 times now but still the brake is too soft.
I was reading that there might be air trapped in the master brake cylinder which I did not touch. Can it be that with running out of brake fluid through the leak in the rear brake line the master cylinder ran empty of fluid?

I don't think the master brake cylinder has a damaged seal but who knows.

I'd be interested in a simple method to bleed the brakes efficiently.

One thing I noticed: if I have the engine running and pump the brakes 10 times in Park I will get the alarm for low pressure (dash light and buzzer) for a short time until the system is pressurized
Question: Is this normal?
If not normal... what is the problem, what can I do?



I read somewhere that I should drive and brake on slippery ground in order to trigger the ABS and then bleed brakes.after.. This process might help to get some air out of the ABS that is otherwise trapped in there.

I just don't understand how the ABS is interfering with normal braking...
In all my old Mercedes cars the ABS is a separate item and won't interfere with the normal brake action.
Here in this Toyota it seems to be different.

I also read about the Toyota Tester that should be used for bleeding but I don't have this Tester/Software/Techstream.

I am patient and wait for your answers.

Please help.

Martin

[riceandpho]

you might want to watch this and ask Timmy, he is also on here, 3rd gen.
Lexus GX470 Brake Booster Replacement - YouTube

try bleeding the brakes again, you need to have the key in "on" position if you are doing the pump method.

you can also try the vacuum method. get 3 big bottles of fluid since its cheap.

[rdruss]

Try the re-bleed, etc. but I wouldn't discount bad orings on the MC plunger.

[AuSeeker]

Look at post number 8 for a PDF of the Shop Manual on bleeding.

DIY: Brake Fluid Change / Bleed Using a Scan Tool

[AuSeeker]

Also read this topic, it maybe all you need to do to get a firm pedal, i.e. "the 40 pumps"

https://www.toyota-4runner.org/1263477-post8.html

[MikeinNH67]

Do you have the year correct? Gen 4 starts in 03.

Good links listed above. You said the brakes were fine before the issue started?

[werminghausen]

Thanks Auseeker,

I read through your link:

I've been through this (with multiple caliper changes) and here is what I found works for me to eliminate the spongy pedal.
Ignore the content in the document about the tester.
It's an electric MC pump so start by bleeding with the ignition ON, engine off. The order is somewhere in the document.
Even with a good bleeding you need to finish with step 2a & b (or 5a). If you have previously done a good bleed then skip right to this step. Do the following with the ignition OFF. They say depress the pedal 40 times but here is where they are short on detail. Forty quick on/off cycles is a waste of time. Do 15 or 20 second cycles. Think of how slow air bubbles travel in brake fluid. Depress and hold the brake for 20 seconds, then let go and wait 20 seconds. Imitate the normal braking stroke as if driving. Repeat 40 times. The pedal may bottom out but hold it there. You will feel the pedal go solid at around 20 cycles but don't stop the cycles there. Do 40 cycles. Give yourself 30 minutes to do this.
Repeat as needed over several days and your brake feel will return.


I still do not understand what the 40 cycles of pressing the brake pedals do.
In my simple mind, if there is air in the system ... simply pressing the pedal does not remove the air.

How can the magic be explained?


Anyhow I did more bleeding today, the first time involving the ign key in on position.

Before this bleeding (3rd time) I drove on my field and got the ABS working a bit (there was some comment suggesting that this would be good because air could get trapped in the ABS??? weird. (there are many teories out there but no one ever studied the system like I would do if I had the documents, electrical and hydraulics...

Anyhow I did the ABS exercise and started bleeding again: Pumps veeery slowly 20+ times in ign off position and the pedal became really hard.
Then in ign on position the booster motor was working for quite some time.

The again 20 slow cycles... the motor came on once in a while.

The bleeding the front brakes.... no additional big air bubbles came out! but maybe I did not see the smaller bubbles?

The I did the rear brakes and here came some minor bubbles at last.

Then with the ign off position my wife (she was doing the slow pumping action) was pressing the pedal again... very slowly, as suggested.

Then I noticed that the rear brakes are not engaging when pressing the brake pedal! The front brake engage however. So there is something tricky going on.


I did test drive after bleeding and the braking has improved for sure.
Is it as good as it gets? I doubt it.


Again all the theories and trying things out are good but having a clear hydraulic and el. diagram how these brakes work would help me.

Thanks Martin

[werminghausen]

Originally Posted by MikeinNH67

Do you have the year correct? Gen 4 starts in 03.

Good links listed above. You said the brakes were fine before the issue started?

Hi Mike,

You are right... I have a 2003.
Brake were done in 2020 with rebuilt calipers and new brake pads (ceramic) and the brakes worked great for 2 years and this year the brake were not so great any more and then I had the incident with the rusted out rear brake line and a failing booster pump.

[MikeinNH67]

Originally Posted by werminghausen

Hi Mike,

You are right... I have a 2003.
Brake were done in 2020 with rebuilt calipers and new brake pads (ceramic) and the brakes worked great for 2 years and this year the brake were not so great any more and then I had the incident with the rusted out rear brake line and a failing booster pump.

Gotcha. If they were squishly before the line failure, do check the pads and make sure they are moving as they should. I had a spoungy pedal only to find one pad was hanging up which made it pull back on the caliper.

[werminghausen]

Originally Posted by MikeinNH67

Gotcha. If they were squishly before the line failure, do check the pads and make sure they are moving as they should. I had a spoungy pedal only to find one pad was hanging up which made it pull back on the caliper.

Hi Mike,

Yes I had dragging brakes in the past as well. I renovated all 4 brakes 3 years ago and they were very good for 2 1/2 years.

Then brake got worse and it might had to do with air entering the system with the slowly failing and later leaking rear brake line. (My thought is it might have drawn air a while before brake fluid leaked out?)

Then came the brake booster failure with replacement of one rear brake line
and the the replacement of the booster motor/pump followed by now 3 bleeding sessions.

During last bleeding session with my wife who operated the pedal sloooowly. I could see that the front brakes were not dragging at all and
to my surprise I saw that the rear brakes would not engage when the pedal was pressed without key. I cannot explain why this is so. Normal brake systems (Vacuum booster) would operate the rear brake of course during bleeding.

Any help is very much appreciated. Please send me the diagrams or whatever Toyota might have in one of their shop manuals about the brakes.

Martin

How little is known about these mystery brakes?

Experiencing a clear improvement after the last bleeding I am tempted to
try this again.

But i would feel so much better if I ciould understand the hdyraulic parts of these brakes.
Who has the knowledge and better knowledge and a hydraulic diagram for the brakes?

Martin

[MikeinNH67]

Originally Posted by werminghausen

Then brake got worse and it might had to do with air entering the system with the slowly failing and later leaking rear brake line. (My thought is it might have drawn air a while before brake fluid leaked out?)
~~
During last bleeding session with my wife who operated the pedal sloooowly. I could see that the front brakes were not dragging at all and
to my surprise I saw that the rear brakes would not engage when the pedal was pressed without key. I cannot explain why this is so. Normal brake systems (Vacuum booster) would operate the rear brake of course during bleeding.

Typically the metal lines will not slowly leak and let air in. They seem to just burst, usually coming to a full stop when lots of pressure is needed.

No vacuum on these systems, you have to have the key on and brake slightly pushed so the pump runs to bleed the air out, mainly the rears if I recall correctly. This may set a brake beep alarm which is annoying. There's also a procedure to pump them up a bunch of times but I haven't found my past problems to be fixed by that procedure. It seems anytime I've ended up with spongy brakes when they were good months before, it is because of a bad caliper or pads that are stuck. Both front brakes were not grabbing? I think I'd focus on that assuming the bleeding process has been followed.

Have you looked at the stickies section for the proper brake bleeding procedure?

[werminghausen]

Thanks Mike,

I had bled the brakes again without any improvement.
I have worked on many brakes but never has such problems.

I am no very keen on working in my unheated garage over the winter without having a plan or knowledge how to deal with these brakes.

I learned to live with these mediocre brakes and avoided driving longer distances.

Now today something happened what could be a bad sign:

All of a sudden I had the brake alarm (the lights on the right side instruments next to the gas level) the light were lighting up and the unbearable sound started, see picture.
As I said I was driving on the highway.... which was a scary moment.

It went away after a few seconds but still it happened.

I have enough brake fluid in the reservoir.
After changing the brake assist motor and accumulator the brakes were working again, my only complaint was that the brakes were too soft and this is probably due to me not being able to understand how the system works and how to bleed it properly despite the many recipes. (again I have not seen one single logical explanation why the bleeding is so difficult).
But I started to live with the bad brake perfromance.

Now the new alarm woke me up because the last time I had this alarm the brakes did not work any more.

This time they came back up after 15 long seconds.

I am very scared driving this car and I wanted to ask for your advise.
Does anyone of you have an explanation of what happened that triggered the alarm?

I notices all the time that in normal traffic after every second brake action the electric brake assist motor was running.


I need help:

Knowing hydraulic systems very well (my profession) I understand that the nitrogen charge of the accumulator might be low and there is simply not enough stored energy to cope with the demand of the brake system.
Is there anyone on this forum who has the specs for the accumulator charge (how much nitrogen charge in bar or psi ).
And has anyone recharged the accumulator.
Is there a Schrader or other charging valve on the accumulator or do
people simply buy a new accumulator?


Best, Martin

[riceandpho]

the front brake bleed, you can do it with the key off, bleed like normal. pump and hold, open bleeder, watch some fluid come out and close the bleeder, let go pedal. repeat like 10 -15 times, should get alot of fluid out, buy a few bottles of fluid since its cheap.make sure the resorvior is full all the way to max line

for the rears, you have the key in the On position, press the pedal down and hold it down, open the bleeder and the fluid flows through automatically.
make sure the resorvior is full all the way to max line.. do it couple times and at 20-30 seconds intervals, youll see some air bubbles if there is air.
start at the passenger rear first, then work on the driver side. i never had an issue, even put in new stainless steel braided lines and saw the air bubble.

you might have to put a block of wood behind the pedal so you dont floor it, i didnt have an issue with that.

[SocalSam]

While applying pedal pressure, crack the lines going to the master cylinder and valve block. Air gets trapped high up in the system.

[werminghausen]

Thanks,

I understand that there must be air in the system somewhere otherwise the brakes would be much firmer.

Although I was bleeding 1 gallon of fluid (I bled the system 3 times) there are no air bubbles coming.

I have only 2 explanations

1) A spot in the system,maybe master cylinder...where the air can hide and cannot be bled by the calipers. (Is this what SocalSam is saying?)
2) There is a possibility that the accumulator (membrane accumulator next to the el. motor/pump at the master cylinder/ABS is leaking nitrogen in small portions and this making the brakes softer And this would also reduce the storage capacity of the pressurized brake fluid.

Is there a testing procedure for the pressure accumulator in the factory manual?

What is the nitrogen filling charge (gas pressure) of the accumulator per Toyota specs?

It is not wise to refill the leaking membrane accumulator even it had a filling valve...

I know of this problem- leaking nitrogen into the system- in the old Citroen and Mercedes hydraulic suspensions. This leads to all sorts of problems in any hydraulic system, aerated fluid is compressible and does not go well with any hydraulic system that is designed around non compressible fluids (hydrauilic oil and brake fluid systems will misbehave).



Best. Martin