Brakes broke!

I'm having this same problem too, except yours seems more serious. My brakes catch at the last few feet and are very uneven. The braking at regenerative braking speeds (above 9 mph?) is ok, but while braking under 10 mph, my brakes stop the car abruptly and the car shudders.

 

I have no answers, but if you put the car in N on a long straightaway (no regen, all friction brakes) and brake, is it worse?

 

There is nothing magical about the friction parts of the system. Hydraulics maybe, friction no. If you don't have a warning light, the hydraulics are probably okay, though the wheel cylinders may need bleeding, which anyone can do. Get to a brake shop as soon as possible, if you can't DIY. Do not drive your cars at highway or arterial speeds until you get them fixed.

OP: Does your parking brake work correctly?

I had a similar problem on another car where grease from a leaking axle seal got on the friction surfaces, and the heat made a new compound previously unknown to man--sometimes gooey, sometimes sticky. That took a new seal, new shoes, and turning the drum to get rid of.

 

Are you sure it's the rears that are grabbing? I'm not disputing, just wondering why you're sure it's the rears.

How many miles, how long since they were last inspected, front and rear? DIY or?

 

if they have Techstream or other higher end OBD2 readers that allow for bleeding of the Prius rear wheel cylinders.

Its possible w/o Techstream/higher end OBD2 reader, but a PITA.

 

When were the brakes last looked at?

It seems like you don't do any DIY, at least brake wise. Its possible the rear brake shoe backing plate, contact points lack grease and are all rusted up. The brake pedal pressure overcomes the rust and give you the "grabbing" sensation.

But, given

you should be seeing the traction control light coming on, and feeling the ABS pedal pulsation kick in. Since you are not stating this, something is definitely wrong.

A brake problem.

 

On a previous car (before hybrids existed), when the rear drums were known to be getting worn, I suddenly had one side wear through the shoe and hit metal-on-metal. That side modulated very poorly, grabbing hard or locking very easily. It was simply a matter of the shoe worn down too far.

 

Sounds very familiar, 'Lectro. I kind of expect it now, but the first couple times it happened, I was tossed
forward in my seat!

Just another Prius oddity...

I would love to see the look on somebody's face, when they rent and drive a Gen4 Prius for the first time.

 

Changing a tire is (ideally) in everybody's DIY repertoire, and if you can manage taking the wheels off, you can manage pulling the drums off (you'll want to pick up a couple M8x1.25 bolts from the hardware store to help them off if they're stubborn). Most everybody has a phone that can take pictures. What's it look like in there?

-Chap

 

Lots of people out there posting similar problems. I am new to priuses but it sounds like there can be issues with sensors at the wheels, electric assist brakes abs and traction control made worse by wet conditions and brakes that don't dry quickly enough. One thing I can say is most suggestions say take it to a dealer for evaluation. Not many solutions posted. So far I have been lucky that my only quirks are an occasional brake reaction that is stronger than my foot pressure should warrant. Are the prius brakes actually brake by wire?

 

Somewhat the reverse.
There is an all hydraulic path from the master cylinder to the slave cylinders in the brakes, which is short circuited when regenerative braking can do the job. With Engine braking adding it's own complexity to the mix. This schematic is one I show to explain why 'bleeding the brakes' is not as simple as other cars, but if you look at it you see that there is always a hydraulic path, but it may lead back to the master cylinder if conditions permit.


When you press the pedal, brake fluid moves. Depending on how well regen is working, it makes it all the way to the friction brakes. So it is not brake by wire, if you press hard enough you always get friction brakes. (the diagram is of Gen 2, but I believe it still represents how the brakes work)

 

The OP (Original Poster) has NOT provided anything useful to go on.

Here is what we can deduce:
1) No DTCs as OP made no mention of warning lights.
2) OP has not even removed BOTH rear drums, to examine the rear shoes, and rear wheel brake cylinders.
3) There is something clearly wrong, but OP is in denial​

A faulty sensor will cause a warning light to be triggered, resulting in a DTC. You make no mention of any warning lights and DTCs.

Drum brakes can be made ineffective by water, if water happens to enter the drum. Only when the water escapes, and the drum and shoes are dry, will braking force be restored. Are you fording deep levels of water that would submerge the rear drum brake? 2ft of water will likely do it.

No.
Read & become informed: How does the brake pedal work? Brake by wire? | PriusChat

 

One way I would try to describe it is as a brake-by-wire system layered on top of a traditional hydraulic brake system.

The regular touch and feel of the system, including control of the power assist, is brake-by-wire. A 'stroke simulator' takes the first portion of the brake pedal travel, transmitting your foot force to the brake control system and that system's feedback force to your foot. This portion is equivalent to the vacuum-boosted power assist portion of previous cars.

Should the above system fail, the stroke sensor should bottom out, and then you are putting your foot pressure directly on the brake master cylinder, thus directly into the hydraulic system. You now have old fashioned manual brakes, without power assist. This is also equivalent to how previous power assist brake systems behave when their assist fails.

Federal regulations require both types must meet essentially identical performance requirements when they suffer system failures and go into back-up fail-safe mode. With the vehicle traveling at a set highway speed, it must stop within a specified (but fairly long) distance when a certain force (500N, metric for about 112 pounds-force) is applied to the pedal. Just like the old days before power brakes, this means essentially putting all your body weight into the pedal.

 

Yea, but I'm talking about my gen 2. My gen 2 is actually scary when I have to panic stop. I agree the gen 4's brakes are touchy, but not as bad as my gen 2. I don't want to replace anything in this gen 2 because it's probably worth $4k and regenerative brakes are EXPENSIVE$$$$.
My situation is hard to explain, but imagine you're applying the same brake pressure at 20 mph or faster until you come to a stop. Once the car brakes to 10 mph, it screeches to a stop. I'm guessing this is when the mechanical brakes, not the regenerative brakes, kick in. It's annoying how I have to modulate the brakes constantly.

 

Mine is worse in the mornings, when I'm going down my driveway at only 1 mph. But when you touch that brake pedal… you better be holding onto something!

But then it also happens in the middle of the day, like if traffic is just creeping along, stop and go, I lightly touch the brake pedal, and sometimes it will abruptly stop.

There goes my coffee… All over my fine white china!

 

I wonder if the issue might be a simple mechanical brake adjustment. I found lots of posts where owners said they found the self adjustment wasn't working correctly and the rears, which should adjust with normal brake usage, were way off. Could be a reasonable DIY task or should be relatively inexpensive labor wise.

Whatever happens please post a resolution if found.

 

I agree someone has worked on these rear breaks and the adjustment is messed up.


iPhone ?

 

A slight correction--the rear brakes on the Prius self-adjust with parking brake use. Older styles would adjust when applied in reverse--these are a little different. That's why I asked in post #4 if the parking brake worked. But the OP seems to have left the room, so maybe we'll never know.

 

Didn't mean to mis-inform anyone. Read a post somewhere last night... and can't find it now. The owner dug into his brakes and actually tested the self adjustment by applying the hydraulic brakes and then the e-brake. He said he could hear the star wheel adjuster clicking when he applied the hydraulic brakes but not the ebrake. Seems like most everyone else says the ebrake does the adjustment. I will have to dig into my brakes soon and will also give it a test or post a link to the information that I found if I can locate it.
Having the adjustment during ebrake use makes more sense why some owners would find that properly maintained brakes get out of adjustment. I am guilty of not using the ebrake often. I think I will get into that habit. Previous owner must have used it. There is paint worn from the pedal.

The OP has not returned, but there are a few replies yesterday from owners suffering this issue. Hopefully they will find an answer soon.