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I had a 2nd-gen Prius that would do this, and I observed it in 2 different instances. The first was when the ABS activated during regenerative braking. The car wasn’t programmed to use the regen brakes for ABS, so it had to switch to the discs, and it would “let go” for a second during the switch. Which is terrifying when you need your brakes bad enough to engage the anti-lock, lemme tell ya.
The second instance is when the traction battery was full, and the car stopped accepting any more charge from the regen brakes. Same thing would happen. Also, when the battery is over-full, it uses the electric motor to turn the engine. It sounds like the gas engine is running, but it’s not; at least, not on gasoline. It’s just burning off electrons.
This second scenario sounds like what you’re describing. What I’m guessing is going on is that the transmission is not disengaging the engine, so the electric motor is turning the engine, which is pushing the car forward, fighting the brakes.
Thanks for sharing your thoughts on it. The van is so quiet and smooth that I'm going a little faster than I mean to be and I especially realize it when I'm getting ready to go into a turn. I'm not putting on the brake hard enough to engage the antilock brakes I don't think, but I'm having to press them a little harder than I would have needed to if I had eased up earlier. I'll try easing up some getting ready for a turn and less braking during the turn and I'll see if that helps.
 

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Thanks for sharing your thoughts on it. The van is so quiet and smooth that I'm going a little faster than I mean to be and I especially realize it when I'm getting ready to go into a turn. I'm not putting on the brake hard enough to engage the antilock brakes I don't think, but I'm having to press them a little harder than I would have needed to if I had eased up earlier. I'll try easing up some getting ready for a turn and less braking during the turn and I'll see if that helps.
I think that is why as soon as the door opens on the Sienna, even before you turn on the engine, the electronics is actually pressurizing the brake system (you can hear it happening if you listen) before you actually have to use the friction brakes...... even before the van physically moves. When you need the friction brakes beyond the regenerative thing, I think there is a little disconnect in the braking between the regenerative and friction brakes and when going down hill, gravity is a hell of a pull as far as speeding up briefly and that is a very unsettling feeling.......just my opinion. Good question for the car nut guy....LOL.
 

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2021 XLE Plus Gray
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I had a 2nd-gen Prius that would do this, and I observed it in 2 different instances. The first was when the ABS activated during regenerative braking. The car wasn’t programmed to use the regen brakes for ABS, so it had to switch to the discs, and it would “let go” for a second during the switch. Which is terrifying when you need your brakes bad enough to engage the anti-lock, lemme tell ya.
The second instance is when the traction battery was full, and the car stopped accepting any more charge from the regen brakes. Same thing would happen. Also, when the battery is over-full, it uses the electric motor to turn the engine. It sounds like the gas engine is running, but it’s not; at least, not on gasoline. It’s just burning off electrons.
This second scenario sounds like what you’re describing. What I’m guessing is going on is that the transmission is not disengaging the engine, so the electric motor is turning the engine, which is pushing the car forward, fighting the brakes.
When the battery is full the energy from the wheels is used to turn the engine like an air pump. It is purely mechanical through the transaxle gearing so it is not burning off electrons. The MG is not involved.
 

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I’ll defer to those who know more than I do about the system. But mine would do it at a stop and show an arrow from the battery going to the engine on the display. That was the Prius, though. The modern systems may well be different.
 

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Based on what I have read, the brake pedal is called a brake simulator by Toyota. The pedal pressure is interpreted by a computer to determine the ratio of regenerative braking to friction braking. Light pressure on the pedal (keeping the ECO gauge in the blue) will result in mostly or all regenerative braking until lower speeds. At 17mph, friction braking is added in to keep some heat in the brake pads (to keep moisture out) and the fact that regenerative braking does not have as much effect at lower speeds. As the speed gets lower, a larger percentage of friction braking is ordered by the computer. At very low speeds (a few mph), mostly friction braking is being done, although there remains a small portion of regenerative braking. (You can hear the whine of the electric motor as the car comes to a stop.) If the computer ever fails to properly apply braking force, there is a failsafe mechanism where pressing the pedal much harder engages a mechanical/hydraulic mode like a non-hybrid has, so the car can be stopped with a failure of the computer. In this mode, it is much harder to press the brake pedal due to limited power assist.
 

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I just picked up my XSE over the weekend. The brakes feel fine and normal. I remember test driving one last December and it had the problem you guys are describing.
 

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I’ll defer to those who know more than I do about the system. But mine would do it at a stop and show an arrow from the battery going to the engine on the display. That was the Prius, though. The modern systems may well be different.
I have noticed with my 2009 Prius that at a stop, if the brakes are lightly pressed, there are arrows moving from this to that, but if I mash (I like that word) hard on the brakes all movement and arrows stop.-- Now, with the 2021 Sienna, I have noticed something similar and interesting. (only 1 or 2 times, so I need to observe further), but I came to a stop at railroad tracks (bummer) and so I decided to press the "Hold" button, but when I looked at the screen for the energy flow I noticed arrows moving from something to somethings (sorry I don't remember what to what). It looked like it was working against itself, so I decided to put it in Park instead and that put a stop to all the arrows, energy flow.
 

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I have noticed with my 2009 Prius that at a stop, if the brakes are lightly pressed, there are arrows moving from this to that, but if I mash (I like that word) hard on the brakes all movement and arrows stop.-- Now, with the 2021 Sienna, I have noticed something similar and interesting. (only 1 or 2 times, so I need to observe further), but I came to a stop at railroad tracks (bummer) and so I decided to press the "Hold" button, but when I looked at the screen for the energy flow I noticed arrows moving from something to somethings (sorry I don't remember what to what). It looked like it was working against itself, so I decided to put it in Park instead and that put a stop to all the arrows, energy flow.
I suspect that you have an AWD. I have found that when in drive and the parking brake is on, power is shown going to the rear drive motor. If I put it in park, power stops going to the motor. Also, if the brake pedal is depressed, the power stops going to the motor, but goes to the motor again when released. You can also feel the power being applied and taken away by the slight changes in the rear suspension, so there is actually power there, not just the display showing something that is not there.
 

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I have the XSE AWD for 2 weeks now with almost 1000 km on the odometer and the brake feels fine to me. I do notice I have to push it a bit more compare to my other non hybrid car but it did not feel weird or anything. I could inch up slowly inside the garage to make it fit just like my other non hybrid car.
 

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We've had our new 2021 Sienna for just about a week now and have loved it! No complaints except for an issue that has come up sporadically and only 3-4 times this week. When slightly braking for long, drawn out stretches there have been several times when the brakes let up. Not completely but it's as if my foot let up on the brake pedal when it didn't. It's happened the most recent 2 times during a turn and I felt like I was suddenly speeding up into the turn, even though my foot hadn't changed pressure on the brakes. Regenerative brakes are new to me so the first few times I wrote it off as being not used to Hybrid Brakes, but I'm confident now something is wrong. It's as if the computer is deciding to switch from Regenerative to conventional braking, and then not pairing the two the the same level. I have no idea if that's what's actually happening, but it's my only thought.

Is anyone else experiencing this? I know Consumer Reports commented on the brakes and mentioned a TSB was issued, but I haven't heard any mentions of this particular situation. They feel over responsive and linear, except for these 3-4 instances where it just let up.

Thanks for your thoughts and advice!
i had had the same issue may times with the 2021 sienna hybrid i have had several months
 

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I suspect that you have an AWD. I have found that when in drive and the parking brake is on, power is shown going to the rear drive motor. If I put it in park, power stops going to the motor. Also, if the brake pedal is depressed, the power stops going to the motor, but goes to the motor again when released. You can also feel the power being applied and taken away by the slight changes in the rear suspension, so there is actually power there, not just the display showing something that is not there.
We actually have the front wheel drive.
 

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We've had our new 2021 Sienna for just about a week now and have loved it! No complaints except for an issue that has come up sporadically and only 3-4 times this week. When slightly braking for long, drawn out stretches there have been several times when the brakes let up. Not completely but it's as if my foot let up on the brake pedal when it didn't. It's happened the most recent 2 times during a turn and I felt like I was suddenly speeding up into the turn, even though my foot hadn't changed pressure on the brakes. Regenerative brakes are new to me so the first few times I wrote it off as being not used to Hybrid Brakes, but I'm confident now something is wrong. It's as if the computer is deciding to switch from Regenerative to conventional braking, and then not pairing the two the the same level. I have no idea if that's what's actually happening, but it's my only thought.

Is anyone else experiencing this? I know Consumer Reports commented on the brakes and mentioned a TSB was issued, but I haven't heard any mentions of this particular situation. They feel over responsive and linear, except for these 3-4 instances where it just let up.

Thanks for your thoughts and advice!
We are having this issue too.
 

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After 500 miles I have noticed the same thing. Not often but sometimes the brake "let up" and the vehicle feels like it's surging forward. It's really disconcerting when you are close to something. Toyota should program the system to transition to the brake pads more smoothly. I'd rather do a brake job 2X more often than potentially run into something!
 

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From my experience it's been when slowing from approximately 30's to 20's, just as when slowing down for a left turn. That's when I've noticed it.
I have been paying close attention to the feel while braking and it has never felt like the brakes were letting up or failing.
 

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From my experience it's been when slowing from approximately 30's to 20's, just as when slowing down for a left turn. That's when I've noticed it.
Same for me on a FWD when turning right. I feel a let up and I push the brakes harder. Sometimes happens without a turn.
 

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Same on my 2021 XLE front wheel drive, had it at the dealership this past Monday and they said they didn’t feel anything out of the ordinary. I tried explaining to them that it feels dangerous at times and that I have a family with 3 children and I use the van for Uber on the weekends and I don’t feel safe driving people around in this condition. I have to take it back this coming Monday and they said they make keep it for more than a day to see if they can feel what I feel. Then one of the service managers tried to assume that it feels different because it’s a hybrid you have to get used to the regenerative braking, I said I’ve owned 3 other hybrids previously and never had this issue. If they don’t fix it I’m seriously going to get in touch with a lawyer I’m tired of the dealership run around BS and just plain frustrated that a $42k car does this. I’ll be straight with you guys if it wasn’t for this brake issue the car has worked and performed flawlessly and getting 38.2 mpg with upgraded 18 inch wheels.
 

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Same on my 2021 XLE front wheel drive, had it at the dealership this past Monday and they said they didn’t feel anything out of the ordinary. I tried explaining to them that it feels dangerous at times and that I have a family with 3 children and I use the van for Uber on the weekends and I don’t feel safe driving people around in this condition. I have to take it back this coming Monday and they said they make keep it for more than a day to see if they can feel what I feel. Then one of the service managers tried to assume that it feels different because it’s a hybrid you have to get used to the regenerative braking, I said I’ve owned 3 other hybrids previously and never had this issue. If they don’t fix it I’m seriously going to get in touch with a lawyer I’m tired of the dealership run around BS and just plain frustrated that a $42k car does this. I’ll be straight with you guys if it wasn’t for this brake issue the car has worked and performed flawlessly and getting 38.2 mpg with upgraded 18 inch wheels.
My dealer made excuses also, i have gotten used to it and always stop sort of slow, but if i had to apply the brakes quick i would be worried. I am looking for a car or van soon to do a trade on and eat the loss and never drive or purchase a toyota again...always loved toyota but risking my life to drive one is not in the cards for me. Soon i will be calling it quits on the brand although i am unhappy about it loved them till this point and was hoping i fix would come but i have not heard of one and not waiting any longer while i am ahead of having a wreck due to this issue.
 
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