Sudden Braking Slow Down in Rain

[clueless remark deleted]

 

I assume that you've owned the car for a few years and that the problem has only appeared recently. If true, then it sounds like your problem happens when the front brakes get wet. If the tech's inspection doesn't show any brake problems, and you are still motivated to take action, I suggest three possible courses of action, from least cost to highest cost:

1. Using your garden hose, apply a strong spray of water to the front rotors and front brake pads to clean off any dirt and debris. Drive your car, get up to 40 - 50 mph, then shift to N and apply the brakes to dry them. Repeat a couple of times.

2. Buy a can of brake parts cleaner. Raise up the front of your car and remove the front tires. Place some newspaper under each front wheel to absorb brake parts cleaner as it drips down. Liberally spray down the front rotors and front brake pads until all dirt is removed.

3. If 1 or 2 do not help, then replace the front rotors and front brake pads.

Also, I am wondering what is the tread depth on your front tires. Since your area usually faces a snowy winter, I suggest that you consider replacing those tires if your tread depth is less than 6/32".

 

I think this may be related to the type of friction material Toyota uses. I have noticed this phenomenon (brakes are much more sensitive in wet conditions) on 2 different Toyota pickups that I have owned and my father also mentioned this when he used to have a Toyota pickup.

I have noticed it on the Prius too. I just try to compensate when it is raining.

 

http://techno-fandom.org/~hobbit/cars/brakerust.jpg
.
It's just a surface rust problem. Regen-less braking [e.g.
Neutral] is a very effective way to clean it off. Do it as
evenly as possible, don't stab at it..
.
_H*

 

In order to effect a smooth stop in any vehicle, you've got to let off pressure a bit on the brake pedal the last few seconds before the car comes to a stop. The reason for this is because as the car slows, less force is necessary to slow it and just before it stops, very little force is required. If you don't let up on the pedal, the car will appear to 'jerk' to a stop. When it rains, the probelem is exascerbated because the water washes the dust off the pads and rotors, making braking more effective.

 

The severity that you describe here makes it sound to me very different than just build up on the break disks. Being this severe (and sudden onset) sound more like an electrical problem. Maybe when the control unit or wiring gets damp (heavy rains) it shorts a bit causing the signal to the breaks to act like you slammed down on the peddle. I'd have the wiring checked for cracks in the insulation or other wear.

 

My 2008 Prius seems to want to jump forward instead of slowing down if I am coasting with my foot lightly pressing the brake pedal and the current speed is about 20 MPH and it is about 40 to 60 degrees outside and the pavement is rough enough to bounce the tires. It only happens rarely, so I am not sure how to duplicate it, but humidity may also be a factor.
Obviously, I have no clue what is going on, but when it happens and an idiot drives right in front of me my heart jumps into my throat as it seems that I have no brakes.

The dealer has not been able to help me, as they cannot duplicate the problem. I presume that also means there were no problems recorded in the cars log.

Can anyone offer any suggestions?

 

NOOOO! Do it at low speed (15-20 MPH) and gently to clean your rotors. Repeat until braking is smooth and without scraping noise.

JeffD

 

This is a traction control issue. It's not a problem, but just an artifact of the regenerative braking system.

At speeds above 7 mph, the Prius uses regenerative braking to recapture some energy. To do this, the front wheels (drive wheels) are used to spin MG2 and use it as a generator. Electricity from MG2 is stored in the HV battery.

When you hit a bump, the traction control system recognizes the loss of traction and forces braking to switch to the friction brakes. The short transition from regenerative braking to friction braking produces the sensation of jumping or lurching ahead. The car doesn't really jump or lurch, but it feels that way since the braking force is reduced for a very short period. Keep pressing on the brake and it won't cause any problem.

Tom

 

Isn't it actually an artifact of the ABS system? The way I understand it, when you hit a bump and momentarily lose traction, the ABS sensor(s) recognize the wheel rotation speed difference. ABS only works with the friction brake system, so the car must switch from regenerative braking to friction braking to activate the ABS. The surge you feel is the transition from regen to friction braking.

 

The "surge" (it's not really a surge) is the transition from regenerative braking to friction braking, but it's not ABS. It's the traction control system. The main purpose of traction control is to prevent the hybrid drive system from destroying drive train components, which is why it is touchier than most traction control systems. Regenerative braking is essentially electric drive in reverse. Losing traction while doing regenerative braking threatens to shock load the drive system, as well as causing a loss of braking action. The traction control system suspends regenerative braking and hands the braking to the friction braking system. At this point ABS can kick in if needed.

I suppose we are splitting hairs by trying to decide which part of the ECU makes the call. Wheel slip is wheel slip, and somewhere in the code it gets picked up and suspends regenerative braking, and as stated above, the reasons are two fold.

Tom