Have assistant press brake pedal. Tool box on pedal works. Use 10mm box wrench and simply loosen the line fitting until fluid leaks out. Do that for all of the lines coming and going from master cylinder / booster assembly. There is nothing Toyota specific about this, cracking MC lines work for vacuum boosted systems as well. Leaking accumulator would cause motor to cycle on frequently. Bad pressure sensor would do the same.

When I did my booster/accumulator swap, I took out the complete assembly. When I installed it, I did not have to bleed at the calipers. I bled the system as outlined above.

I don't think there's much here for knowledge base when it comes to the inner working of the master cylinder/pump setup. You sound like you may know more than most on how it works. Any thread I've read on these, the op just replaced the whole unit. That brake beep with lack of brake feel is surely scary and to me would point to the master cylinder system assuming you have no leaks and loss of fluid. Almost like a seal dislocated allowing no pressure. I guess it's possible an air bubble in just the right place could make this happen. I think if it were mine, I would start capping sections off to see if you can make the squish go away but no brakes is concerning.

My system never lost braking power so the fix was crystal clear. Loss of brake pressure is another issue with several causes. True, bad master cylinder seals would lead to soft pedal and no braking power. Accumulator could be bad. Pressure sensor could be had. And the system may still have air. Start with the easiest which is bleeding.

Hi SocalSam and all.

There are 2 separate issues.

Issue #1 'soft' braking -should be air in the system.
Hell knows why bleeding is so complicated.

Issue #2 motor/pump is coming on every second time I hit the brake and
this does not seems right.
reason could be the accumulator low on nitrogen.

#2 also can lead to the situation in the extreme when the brake must be hit several times hard and the booster has not time to build up and store pressurized fluid...so pressure drops below the minimum and the alarm goes off.


I was driving very slowly the last days because of the alarm issue (I need the car and it is too cold right now to work on the brakes).
When I am driving defensive there is no alarm.

Another test is the car in park and I hit the brake several times... say after 5-10 pushes on the brake the exact alarm goes off.... after 15 seconds it is silent again....as the pressure recovers.


What is happening inside the system. I was thinking..... if there was no air in the the lines (as it should be) there would be almost no flow of brake fluid in the lines at all (because fluid is not compressible) and this less pressure loss during braking? meaning the accumulator would last longer?
I am not sure about this but it might be this way.

The other option would be that there is simply not enough pressure stored... in case the accumulator lost tpp much gas (which is typical for membrane accumulators).

This is the only connection between #1 and #2 if at all true.

For #1: soft braking: I have bled the master cylinder.... when I installed the rebuilt pump and accumulator.
Normally after the MC build up pressure and pump.... then the bleeding is all up to the ends of the 4 lines: bleeding at calipers. Any remaining air from the MC would then come out at the end of the lines after enough fluid is pumped through.

I have not revisited the MC bleeding for that reason... but you said there might still be air trapped that would not come out at the calipers?

See if air from the MC would come out by bleeding the MC (cracking the 4 lines to the 4 calipers) then the air would come out as well at the calipers.
Or did I miss something?


dragging calipers:
I know that one of the T4R issues are dragging calipers.
I have rebuilt the calipers maybe 4 years ago and the brakes were great after that.... and the brakes have become increasingly softer since then.

This means I had the soft brakes way before my initial incident last fall with complete failure of the brake booster during the event when one of the rear brake lines failed (rusted out rear brake line). I repaired the brake line and installed a rebuilt booster motor/pump/accumulator.

Could there be a third issue with the brake calipers causing 'soft' braking?
Maybe it is not the 'air' after all?

Sorry for the lengthy text.

Martin

Friend, don't over-think this. If you performed gravity feed, air will not push through and out of the calipers. Air rises and gets trapped at the MC which would cause pump to cycle constantly. It takes about two minutes to crack the MC line fittings. I take it that you are in the snow belt? After the first MC line bleed, drive in the snow and trigger traction control and ABS events to get the valve block to cycle. Air can get trapped inside ABS valve block. Repeat MC line cracking. Anyways, there is some good news here. Pressure sensor appears to be working. And system does eventually build up pressure which also means accumulator could be good.

Update on accumulator.

I came across the Pajero accumulator youtube.... It is a piston accumulator.

I am sure the Toyota accumulator is very much the same.
I read that someone was saying this is a membrane accumulator....
it is not!
This youtube is saying that the nitrogen charge is 1100psi = 76 bar.
Assuming this is correct... the Toyota will be very much the same.

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


I actually know how to rebuild the piston accumulator and install a valve on top in order to recharge the 75bar.

Martin

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Thanks,
Yes if there is an air pocket somewhere that I did not bleed in the MC then you are absolutely correct.

Yes I am in the Boston area, cold right now.
So the plan is to not run the el motor (no key) and bleed the AC manually by depressing the pedal, correct

Would you start with
1) opening the pressure line off the accumulator (the winding line with red protection from accumulator into the MC) and bleed until fluid only comes out?
However there will be residing pressure from the accumulator...Should I bleed with the accumulator pressurized or drain the accumulator first - stepping on the brakes and take off the accumulator pressure
2) close that line

3) Open the 4 lines, each at a time, certain order?

The do the ABS braking and bleed again 2) and 3) (1 not needed again).

Thanks again, Martin

I like the idea of forcing the ABS to kick in in case there's air in there. Some GM's will have a soft pedal until you do this. Standard bleeding procedure is to start with the caliper furthest from the master and work your way to the closest one. According to the bleeding sticky, you need the key on and pedal partially depressed for the rear brakes as the electric motor pushes the fluid out. Obviously keep the level topped off. When I replaced my lines, I followed those directions but still had a soft pedal. I found my front calipers were toast. Replaced those and followed the procedure again with no issues. Point is, don't assume the calipers are still good. Some folks seem to be changing them on a 3 or 4 year rotation, especially in the climate we live in.

Hi Mike,

SocalSam was suggesting to bleed the MC, not the calipers... my questions for MC bleeding were 2 posts back,
I 'll try and bleed the MC (one input line and 4 output lines at MC).

Input MC must be done with the accumulator under pressure, guess ...And probably so is the rest of the 4 out lines done (with the accumulator under pressure).

But I wanted to ask again if this is correct to be done with the key in position 2 (before engine start which runs the el pump).
There could be a lot of fluid be sprayed around.... a potential mess... I can imagine.


For the calipers I totally agree. They are a bad design if they fail that early but what can we do. Changing them is the only safe cure.
And I don't know if they contribute to soft brakes or if these are unrelated issues.

Best, Martin

maybe there is a need to hook up the toyota techstream scantool and check for those error codes. that may help with more in-depth in diagnostic.

have you look at the repair manual?

did you use your vin number to buy the replacement parts?

Martin, you mentioned the 4 corners in your numbered steps so that's why I listed what I did. Bleeding at the MC would be messy if just cracking the lines near it. Some folks route tubes back into the reservoir to make easier. I would think you'd need to run the pump for MC bleeding too but this is beyond my knowledge for this particular system. Either way, I'd want to bleed to the calipers after breaking open any connection closer to the MC. One other thing I've heard but never seen is that old hoses could make for squishy brakes. I replaced my rears because they were looking tired.

Something else you could try would be to pull your calipers off the rotors, compress them completely and put some blocks of wood tightly in between the pistons to see if squish goes away. Just throwing ideas out there to narrow things down.

I did mine with key on engine on. Used a tool box to apply pressure to brake pedal. Then 10mm box wrench to loosen the fittings just enough for fluid to weep. It should only take a second or so cracked to get a result. Yes, some brake fluid will escape so swaddle with shop towel before starting. Once you are done, brake pedal should firm up. Then find some snow or wet leaves and trigger ABS and Traction events to get the air moving through the valve block. If pedal goes soft again, redo MC bleed. Go for it and let us know how it went.

Thanks SocalSam and Mike,

I'll do

-install new Accumulator- (to eliminate this part from the equation)
-MC bleeding followed by ABS braking and another MC bleeding-
-install new calipers in front-

I'll do the accumulator and MC bleeding first and report back.

Thanks for all your help.

Martin

I did the first round of MC bleeding before I do accumulator exchange or caliper exchange.

With key in on-position and my wife pushing the pedal.
I cracked the input line (that red snake line where it enters the MC)
Then I cracked the 4 output lines, rear then front (not sure if the order has any relevance).

I had plenty of paper towels and watched the reservoir level closely.
It was not very messy.

I did not notice any difference in pedal firmness I fear but I'll watch for a couple of days and do some ABS braking, then repeat the MC bleeding as it is simple to do.

Martin

How is stopping power? Any brake fade? Pedal feel is medium to light and feels "Japanese" which is more finesse than German firm. (I'm going to catch heck but firm is how my Merc C55 brakes feel.)