Brake pedal sinking Toyota Sienna

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Ok guys,
I have a strange problem for some time. For several months in the morning or after 5-6hrs rest, brake pedal on 2015 Sienna is super soft, and barely stops getting out of garage on 7% grade. 2-3 braking attempts and things go back to normal. Thought first worn pads, as I sucked out any life out of them, installed Pagid pads, Element3 rotors, but same thing. I kept forgetting but last two weeks I was driving car more as I was upgrading suspension on BMW. So yesterday I decided to check pads as I was switching from winter to summer tires. In addition to discovering leaking strut, everything on calipers looked normal. All springs, retainers etc are there. No loss of fluid. Anyway, I decided to replace all hardware on pads, but nothing. Today same thing.
At this point I am suspecting that it is master cylinder. But, what is confusing me is that brakes are normal after few braking attempts. Actually, these Pagid pads are super responsive, nothing like mediocre OE ones.
Anyone? Trav? Critic?
 
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Press brake pedal for three minutes. If the pedal doesn't sink, your master cylinder is likely still ok and you should check your brake booster and corresponding hoses for leaks.
 

edyvw

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Press brake pedal for three minutes. If the pedal doesn't sink, your master cylinder is likely still ok and you should check your brake booster and corresponding hoses for leaks.
No leaks. Fluid is at max line since I replaced pads and flushed fluid few months back. In the morning, pedal just sinks, barely stops car.
 
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It could be the master cylinder, but is just as likely to be the mechanical section ( not the electronic section ) of the ABS control section if it has ABS ( I am not familiar with that vehicle ). Look for a setup with 4 lines going off too all 4 wheels. It is the aluminum section of this system that fails if the brake fluid is not changed often enough and enough water is absorbed into the brake fluid from ambient air. Brake fluid is highly hydroscopic ( will pull water molecules right out of the air from moisture in the air ). And when enough moisture gets into the fluid bad corrosion happens. Another problem that can happen is copper gets into the fluid and copper and aluminum are at opposite ends of the electro-negativity chart, and if there is copper in the brake fluid it will react like a battery with the aluminum in the ABS section and corrode the aluminum.

So there are two reasons to change the brake fluid every couple of years. Copper and water contamination. There are test strips you can buy on Amazon to check for copper levels, and there is an electronic pen with LEDs to test the percent of water in the brake fluid.

Basically, after you fix the problem, flush the system good so not old contaminated fluid is still in it.
 
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It could be the master cylinder, but is just as likely to be the mechanical section ( not the electronic section ) of the ABS control section if it has ABS ( I am not familiar with that vehicle ). Look for a setup with 4 lines going off too all 4 wheels. It is the aluminum section of this system that fails if the brake fluid is not changed often enough and enough water is absorbed into the brake fluid from ambient air. Brake fluid is highly hydroscopic ( will pull water molecules right out of the air from moisture in the air ). And when enough moisture gets into the fluid bad corrosion happens. Another problem that can happen is copper gets into the fluid and copper and aluminum are at opposite ends of the electro-negativity chart, and if there is copper in the brake fluid it will react like a battery with the aluminum in the ABS section and corrode the aluminum.

So there are two reasons to change the brake fluid every couple of years. Copper and water contamination. There are test strips you can buy on Amazon to check for copper levels, and there is an electronic pen with LEDs to test the percent of water in the brake fluid.

Basically, after you fix the problem, flush the system good so not old contaminated fluid is still in it.
Very unlikely.

I never change my brake fluid every couple years and never have this problem.

What you state seems plausible on the surface but the fact is, several manufacturers do not spec changing fluid every couple years (don't at all, using same brake fluid dot spec) and they would in a heartbeat if a critical system like braking was at risk. It would be a huge recall and lawsuits.

Besides it is a 2015, even if what you suggest eventually happens (in more like 20 years), it's going to take FAR more than 6 years.

Accept the truth, that most people don't change their fluid every two years and there is unquestionably not an epidemic of this happening on mere 6 year old vehicles. It's absurd to even pretend that is true.

The most likely problem is that the brake booster manufacturer got ahold of some defective seals and they've degraded prematurely, nothing to do with a small % water or copper. 2nd most likely is the master cylinder wasn't machined properly and roughness wore the seals.

It is absolutely not water in the fluid. That causes the sinking pedal when you are pushing the brake system hard from aggressive driving or long downhill braking, and elevated temperature causes the water to boil and create steam in the line.

However at the same time it is worth trying to bleed the brakes now that this situation exists. Maybe there was a prior excessive heating event, so then it's like changing your tranny fluid if you have an excessive tranny overheating event, or engine oil, but the root cause was not merely not changing the fluid. Hygroscopic or not, 6 years isn't going to do this to a sealed hydraulic system unless it's got way more miles/wear on it than usual.
 
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edyvw

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It could be the master cylinder, but is just as likely to be the mechanical section ( not the electronic section ) of the ABS control section if it has ABS ( I am not familiar with that vehicle ). Look for a setup with 4 lines going off too all 4 wheels. It is the aluminum section of this system that fails if the brake fluid is not changed often enough and enough water is absorbed into the brake fluid from ambient air. Brake fluid is highly hydroscopic ( will pull water molecules right out of the air from moisture in the air ). And when enough moisture gets into the fluid bad corrosion happens. Another problem that can happen is copper gets into the fluid and copper and aluminum are at opposite ends of the electro-negativity chart, and if there is copper in the brake fluid it will react like a battery with the aluminum in the ABS section and corrode the aluminum.

So there are two reasons to change the brake fluid every couple of years. Copper and water contamination. There are test strips you can buy on Amazon to check for copper levels, and there is an electronic pen with LEDs to test the percent of water in the brake fluid.

Basically, after you fix the problem, flush the system good so not old contaminated fluid is still in it.
That won't happen in my vehicles. Toyota and VW get flush every two years, BMW 4-5 times a year (tracking vehicle).
Highly doubt ABS. Now, I never exclude anything, as there is always the possibility of some ridiculous failure. But, moisture in brake fluid is not the culprit 100%.
That makes me think. This was a thing in 1990s when DOT 4 fluid started to become a thing in European cars. Older cars would blow some rubbers when DOT4 was used. I use in all three vehicles ATE TYP200 which is track capable fluid. One side of my brain is wondering whether such fluid would cause an issue? Another side is telling it is 2015, 77k miles, and fluid DOT4 should not be the culprit behind the failure of some component.
 
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There are a few ways to get to the bottom of it depending on what tools are available.

How I would diagnose this in the shop is I would use a capable scan tool and command all 4 isolation (inlet valves) in the ABS ass'y to close and then hit the brake pedal. If the pedal is now firm and does not sink the master cylinder is okay. If the pedal sinks the master cylinder is bypassing or there is air between the master and abs.

If the pedal is firm with all isolation valves closed open the valves and close one at a time. If closing one valve changes the pedal feel that circuit is the one with the issue. The issue is either air in the circuit, a leak in that circuit, or a failed dump valve in the ABS unit causing fluid to move into an accumulator in the ABS instead of moving to the wheel. If closing one at a time doesnt change anything, do 2 at a time. Its possible but unlikely that there are 2 circuits with issues.

Lacking a scan tool you would need blockoff plugs to manually isolate circuits to determine which circuit has the issue. It can be done but is far more time consuming.
 

edyvw

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There are a few ways to get to the bottom of it depending on what tools are available.

How I would diagnose this in the shop is I would use a capable scan tool and command all 4 isolation (inlet valves) in the ABS ass'y to close and then hit the brake pedal. If the pedal is now firm and does not sink the master cylinder is okay. If the pedal sinks the master cylinder is bypassing or there is air between the master and abs.

If the pedal is firm with all isolation valves closed open the valves and close one at a time. If closing one valve changes the pedal feel that circuit is the one with the issue. The issue is either air in the circuit, a leak in that circuit, or a failed dump valve in the ABS unit causing fluid to move into an accumulator in the ABS instead of moving to the wheel. If closing one at a time doesnt change anything, do 2 at a time. Its possible but unlikely that there are 2 circuits with issues.

Lacking a scan tool you would need blockoff plugs to manually isolate circuits to determine which circuit has the issue. It can be done but is far more time consuming.
Yeah, do not have a scan tool like that.
I was wondering about air. The pedal does exhibit spongy behavior, and then sinks, but only in the beginning. Like I said, after few breaks, things are normal. With air ,IMO spongy feeling should be there all the time. Plus, I pressure flush system a few months back (the issue was before flush) and nothing changed. I purposely ran 2ltrs of brake fluid just in case, but pretty much the same behavior after, with a new rotor and pads.
 
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Almost 100% chance it is the master cylinder.
I had the same thing happen to me on my 1994 Accord, it would sometimes sink to the floor, and then on another time be basically normal, the rubber seal was failing over time. Put in new MC and problem went away for good.
 
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Although I agree with the other here that it's most likely the master cylinder however, have you tried a re-bleed of the brake fluid at each wheel? Just curious and I may have missed it in your post/replies.

I've owned Toyota/Lexus vehicles and I've always(until recently) hated their mushy/soft brake pedal feel when they were new let alone as they age or if the MC is on its way out.
 
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My first car experienced this. Pedal would sink only if I applied gentle pressure, like slow, stop & go traffic. Hard pressure, it would remain firm. No leak, just bypassing. Swap out the m.c.
 
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I noticed our 2018 Sienna pedal felt soft when the rear pads were seized in the caliper bracket. I had to beat the pads out with a hammer because they wouldn't slide. To fix it, I took the caliper bracket to my bench grinder and ground away all the rust until it was smooth. I would check to make sure your pads are sliding smoothly.

If you want to get some fluid of of the ABS without a scan tool, I'd find a safe place and jam on the brakes to get the ABS going.

We like our Sienna but man that thing always has some kind of problem with the brakes. I think the brakes first started having problems at 12k miles. I just recently put new Brembo GG pads on the front and it seems to stop much better.
 

edyvw

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Although I agree with the other here that it's most likely the master cylinder however, have you tried a re-bleed of the brake fluid at each wheel? Just curious and I may have missed it in your post/replies.

I've owned Toyota/Lexus vehicles and I've always(until recently) hated their mushy/soft brake pedal feel when they were new let alone as they age or if the MC is on its way out.
Yeah. It started few months back. I thought I just pushed pads too much, and they were to minimum (not to plate carrier). Swapped pads and rotors, flushed thoroughly all wheels. Same.
 

edyvw

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I noticed our 2018 Sienna pedal felt soft when the rear pads were seized in the caliper bracket. I had to beat the pads out with a hammer because they wouldn't slide. To fix it, I took the caliper bracket to my bench grinder and ground away all the rust until it was smooth. I would check to make sure your pads are sliding smoothly.

If you want to get some fluid of of the ABS without a scan tool, I'd find a safe place and jam on the brakes to get the ABS going.

We like our Sienna but man that thing always has some kind of problem with the brakes. I think the brakes first started having problems at 12k miles. I just recently put new Brembo GG pads on the front and it seems to stop much better.
Nothing stuck. But I agree, brakes are always an issue. I had always issues with undersized brakes in Land Cruiser Prado, so as soon as I got Sienna I switched to EBC brakes as vehicle started to exhibit same issues my Prado did (vibrating rotors). It was dramatic bump in performance, but I did wear them out in 30k, pads and rotors. Will see today, I might just get new MC and change it and see what happens.
 
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