Thanks for asking about the firmness.

I believe the MC piston is tight, brake is not fading. Once the pedal it hit the braking is constant and pedal is 'fading'.
However the pedal feels soft in general and I don't like it. But there seems to be not much I can do about it with bleeding.

When I first bought this car the first thing I noticed was the soft brakes and I worked on the calipers in 2020, all of them. I noticed that pistons were seized so much on the front brakes that I had to replace the front calipers while the I could just reseal the rear calipers.
After that (calipers were all free with new pads) the braking was very much improved and firm.

Two/three years later the brakes were getting softer and softer and then in Year 2023 the rear brake line broke and the pump failed and left me stranded. This was not fun as I had to tow the car back home.
After repairing the broken rear line and installing a rebuilt brake motor/accumulator the brakes work(kind of) but is still too soft.
Plus I have the problem that after every second minor braking action the motor/pump is running. This at least once lead to a difficult situation on the highway when after some harder braking the brake alarm went off for about 15 seconds .
I still don't know if this is normal and how often others brake until the motor has to replenish the pressure- therefore my questions around the accumulator, no one said something.
I know as a fact that the accumulators (piston and membrane accumulators) can be leaky and nitrogen atoms can escape leading to lower fluid storage capacity and gas in the brake fluid... leading to sof brake pedal just like air in the system.
In this case the gas would then probably stay within the MC area....

Without having more success with bleeding I guess my next bet is working of the calipers.


Japanese brake feeling.... True, I was driving BMW and MB back in the days on the German Autobahn in the 80 and 90s and You could definitely not do this kind of race driving with my T4R. That might be a DNA issue- the car is simply not made for that performance. However my T4R brakes were at least factor 1.5-2 better after my last rebuilding in 2020. So I know if can be much better.

Any of your opinions would help regarding the intervals the brake motor should kick in.
And the effect of dragging brake calipers on braking and pedal softness.

Martin

It's normal for the ABS/master cylinder booster motor to kick on about every 3 depressions of the brake pedal and stay on for about 15 seconds max.

It kicking on every 2 depressions is not normal and it's losing pressure somewhere IMHO.

Thanks Auseeker,

That is the first answer indicating that something is not correct with the pressure storage?

it could be A) losing pressure from the accumulator..... or B) the pressure storage capability of the accumulator is
compromised/reduced (not enough energy is stored)
Or C) the stored pressure gets exhausted faster than it should during braking... can this happen with a seized caliper piston?

With A) the pressure storage would be normal but pressure is leaking from the accumulator (I don't know if there is a possibility that this can happen).
A leak in the accumulator would however lead to a drained accumulator at every start up of the engine... but this definitely does not happen.

So I believe I might have problem B) or C)...

Thanks for any help.

Martin

My booster motor runs in a similar pattern. When on long freeway runs, motor does not run at all.

Does OP have other T4R owners near him that will let him drive their trucks and vice versa? I think experienced owner will be able to tell if something is not right fairly quickly. If there is no fading and no unexpected changes in brake pedal feel, it could be hardware is OK and OP's expectation for brake pedal feel is different from his German car experience. My '05 C55 brake pedal is firm and does not have nearly the travel as my T4R. I am not alarmed at my T4R's relative softness because the brakes have stopping power, are easy to modulate, and do not fade. There are no unexpected changes in brake pedal feel. I don't want to diminish the importance of getting the hardware right but I wonder if personal preference is in play.

As I stated a couple of times-
I had the same car brake opeerating much, much better after I replaced all calipers.
I know also that pedal firmness can be better (even though it is no comparison with the German cars I have).

SocalSam is correct it is simply the braking power that counts not the absolute firmess ( I have a 996 Porsche and that brake is like a rock). I am not hung up with the absolute firmness of my T4R.
I think that the effort to get the car to a halt is just too much and I cannot drive faster at all due to this condition, it is not safe.

The incident on the highway when the alarm went off during fast driving was very alarming to me.

My only explanation for the alarm during driving is that this had to do with the fact that the motor/pump could not replenish the pressure level as I had to do 2 sudden brake pedal pushes and this triggered the alarm.

Can I ask: Who had a similar experience with their T4R- alarm going off on the road that went away after 15 seconds (no failure of the motor)?

This makes me very suspicious and therefore I am not giving up finding answers and solutions.

My plan is
1)rebuild the front calipers and then test drive, see what happens.
2) install a brand new accumulator and check in again.


How would you proceed?

Martin

In your very 1st post in this thread you stated...

"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)."

My question is what year is your T4R actually, a 2002 or 2003, I ask because a 2002 is a 3rd Gen not a 4th gen?

That being asked, at this point I would replace the entire master cylinder/booster assembly, a bit pricey at about $1300 to $1350 +/- depending on source but it should solve all of your issues provided you get all the air out of the system after installing the assembly.

Link for the assembly for a 2003 T4R.


47050-60080 Genuine Toyota Brake Booster Assy, W/Master Cylinder

OK, I'll accept OP is not happy with his hardware. OP's no confidence is clear. Before getting into the hardware, complete bleed to make sure air in lines is eliminated. Caliper seal is possible. Getting any of the bad caliper seal symptoms? Research caliper seal test. Lift car and look for piston retraction. Master cylinder seals would cause similar soft pedal but not individual rotor and wear issue. Leaking accumulator is also a candidate. There is a test in which pedal is pumped with engine off to test accumulator. I saw it here but I can't find the link. IIRC, the result for good accumulator is pedal going hard after a certain number of pushes. If pedal stays soft, fluid is getting past a seal somewhere. My memory is bad so do not rely on me.

On a side note, I have full confidence in my brake system which comes from being able to pull used parts and installing them. I get that this approach is not for everyone. A big part of my confidence is knowing I don't have to rely on mechanics and spend the big bucks on new parts. Anyways, our trucks are machines so there is an answer.

I did exchange the broken el pump/accumulator with a so called rebuilt unit for $300 on ebay last fall (i fixed the broken rear line but the system still did not work- the pump was dead). The rebuilt unit worked in a way that my dash light and sound were gone. Since then I struggle with the brake performance, try to bleed etc.
As a side observation I realized that the brake motor came on more often than I remembered and then the alarm on the highway.

I believe the soft brake and the accumulator are possibly unrelated issues as I had the soft brakes before the incident last fall but I had not the motor coming on so often (every other pedal action).

Anyhow I believe the accumulator could be suspect.
So I did test the old accumulator which I replaced with a reman unit.
(heck knows what they sold me as a reman unit... as it performs worse that the old one I removed).

So I did pressure test this old accumulator and it had 60 bar of nitrogen charge.

Then I tested the brand new accumulator I bought lately and it measured 75 bar charge.

This means the original accumulator lost 15 bar compared to the new one.
This is not unexpected and 60 bar (for a 75 bar accumulator) is probably the limit and should be exchanged.

So now the question is what the so called reman accumulator measures...
Its installed now and I cannot test it...but I will remove it some day and test.

Again, I think the accumulator has an influence on the amount of energy stored (how often the pump kicks in) but it should have no influence on the softness if the brake.

pictures attached. I machined a fitting to hock the old and new accumulator up to the hydraulic test pump.





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Attached Images

     

Seized front caliper: I can feel that my T4R is pulling to one side when braking harder... a sign for a dragging caliper.

another idea came up.
The ABS is probably part of the equation here. Can a contaminated ABS modulator cause a soft brake? There are probably internal solenoids and valves... debis might cause malfunctioning of the brakes in general?
Has someone taken the ABS apart for cleaning purposes?

Martin

I'm learning more about our brake booster system in this thread so I feel like I am replying for my benefit more than for our OP. My apologies. When I got my 2003, it pulled hard to the right at freeway speed with sudden brake application. I learned how to drive around this which gave me time to look rotors over. I did not see uneven wear. And then my booster pump failed making this awful noise:

https://www.youtube.com/watch?v=EbhZzsfan6o

I never lost powered assisted braking so bad motor bearing was the obvious diagnosis. I replaced the motor and accumulator as an assembly which solved the problem. Unexpectedly, the pull to the right went away with the bad motor. I had the brake assembly our of the car and most of the brake fluid drained out. I can only guess that debris was blocking the filter screens and drained out with the fluid thus curing the pull.

One more observation to offer. With engine on and car in park, I stabbed the brakes to see when pump would come on. It was once every three applications and for about 5 seconds. From the many posts I have read on line, this seems to be a common experience. Owners report no braking issues even after years of this.

[QUOTE=AuSeeker;3830693]In your very 1st post in this thread you stated...

"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)."

My question is what year is your T4R actually, a 2002 or 2003, I ask because a 2002 is a 3rd Gen not a 4th gen?

That being asked, at this point I would replace the entire master cylinder/booster assembly, a bit pricey at about $1300 to $1350 +/- depending on source but it should solve all of your issues provided you get all the air out of the system after installing the assembly.


Hi Auseeker,

I misspoke, I have a 2003 T4R with a 4.7 V8. So I have this diffcult to maintain electric motor assist Brake/ABS unit that is not well understood (by myself) as I simply know the old system with the idiot proof vacuum booster and add on ABS of my older Mercedes.

As I know hydraulic systems very well (I am a mechanical engineer) i would need the hydraulic diagrams or true sections of this unit and some data in order to understand the functions, maintenance and repairs better.
I believe I could do a much better job this way and don't ask stupid questions.


For sure I could buy a new unit and install and most problems would be solved... but I love these mechanical challenges and I am happy to drill into it and learn.

I'll look into the manual excerpt.

Thanks, Martin

Yes I can confirm a similar experience when my brakes worked well in the past.

Right now the bake motor comes on at every second application for about
15 seconds.

It is not only that the motor comes on more often but also much longer each time.

This could indicate that my currently installed ' rebuilt accumulator' is very weak.
An accumulator with lower charge needs more oil volume pumped into the accumulator in order to reach the system set pressure.
So far this is my theory.

I reached out to the place where I bought the rebuilt unit on ebay and asked the question to what pressure they were charging the accumulator they sold me as 'rebuilt'..... no answer so far.
My guess is they did not do anything to the old accumulator because the filling valve on top still has the original old painted seal.... they simply sold an old accumulator as 'rebuilt'.

Martin

[QUOTE=werminghausen;3830771] I wish you the best of luck trying to understand how these ABS booster master cylinder systems work on these T4Rs, if you figure it out please enlighten the forum!!

That being said even Toyota dealership/Techs know very little on how this system actually works, if you take yours to a dealer they WILL NOT replace any of the parts like the accumulator/motor/pump, they will only replace the entire master cylinder booster assembly, there is nothing in the Toyota Shop Manual on servicing this system other than the bleeding process and how to R&R the complete assembly.

We even have a couple Toyota Master Techs as members here and they say the same.

How the actual master cylinder works is basic hydraulics, but it's the ABS booster portion that is the mystery and I think that lack of available knowledge is intentional on Toyota's part!!

That lack of knowledge could be because of possible liability issues for Toyota is my best guess since it's the brakes.

If I suspected a bad accumulator, I would be in there replacing. I kept my recently removed booster/accumulator and I have a spare that I bought cheaply at the junkyard.