Gen 3 Rear Brakes

[kevin h]

Getting a bit worried about the binding rear brakes.

Normally after washing the car, and parking her in the garage, there is a small amount of bind when I pull off the next morning , but nothing to worry about.

Yesterday, after standing outside on a wet and windy day, and being parked in the garage overnight, when I tried to pull off in the morning it was as if I had forgotten to put the car into Drive.

The brakes held until I put my foot down and the ICE engaged.

Shouldn't be happening, should it.

Will have to mention it to the dealer on the 30K service and hope its nothing serious.

[Grumpy Cabbie]

The rear brakes can get very little useage on a Prius because of the regen system. If you don't do many miles they can rust up quicker than perhaps they should. They're easy to see so if they're getting a little rusty you could always - and do this when nobody is behind you! - slam the brakes on hard so as to override the regen system and remove the rust from the discs. Another way is to go down a steep hill with your foot on the brakes. I also believe if you put the car in neutral and brake this will not use the regen system and only use the friction pads thus clearing rust on the rear discs, though technically it could be illegal to engage neutral with the car in motion.

I think you get the idea. If you're gentle on the brakes they can rust up quickly.

(a simplified explanation of the regen system is that regen braking (ie not using friction pads) occurs with light braking above 7 mph. So gentle braking for bends on a 30 mph road doesn't use the friction pads to any great extent if at all. Hitting the brakes a bit harder or using them below 7 mph causes the friction pads to engage. One benefit is your friction pads - traditional brakes - last longer than on normal cars. One downside is the brakes can rust up quicker if you don't do many miles).

[timberwolf]

As regenerative braking only operates on the front wheels, can anyone explain why this would cause a problem on the rear brakes please?

I've always assumed that light braking at the rear is normal i.e. most of the braking force is at the front with enough braking at the rear to prevent the rear of the car trying to overtake the front of the car (stepping out, spinning, etc).

Does the Prius have more problems with the rear brakes than other makes/models? Could this just be down to bad design, e.g. slide pins more prone to getting stuck, stuck pistons,... or just that the brake discs are somehow more exposed on the Prius?

[Grumpy Cabbie]

Most of the braking over 7 mph is done on the regen system, which as someone correctly pointed out only applies to the front. If you do lots of gentle gradual braking over 7 mph then you will only be using the regen system and not the friction brakes front or rear. Though there is a very complicated sliding scale of brake pressure v speed for the mix of regen to friction braking if we want to get fully technical, but generally if you don't brake hard then your rear discs won't get used. You can't really use comparisons to traditional cars braking systems.

Put it another way, I have 63k miles of hard taxi use on my Prius and have gone through one set of front discs and two sets of pads, yet the rear pads and discs are hardly worn. Is this a design fault? I don't believe so. I find the Prius brakes to be very responsive and sharp if needed and very strong in emergency situations - esp when the emergency braking system thing kicks in - boy do you know about it when it does! Some people like the fact that the brakes last much longer than normal cars and that the regen system recycles energy that is normally lost, though drivers who do limited miles or brake gently might experience corroded rear brakes. I just can't see that as a problem. I'm sure some folk complain they're advised to replace tyres after 6 years despite them having good tread on, as they only do 2k miles a year etc.

[timberwolf]

So if understand what you are saying, because of the regen at the front, the system holds off applying friction to both front and the rear? (i.e. it can't hold off applying friction at the front but apply it to the rear?) Therefore compared to a conventional all-time friction system, the rear brakes are applied less (because the system is busy using the regen brakes at the front). So it follows that if the Prius can so sparingly apply the friction brakes at the rear, then in a conventional car (as trade off in the design) the brakes are generally being applied too much (but causes less rear brake failures?)?

As for design fault, anything that requires an extra trip to the garage because something has broken or has seized is a fault in my book.

[kithmo]

Therefore compared to a conventional all-time friction system, the rear brakes are applied less (because the system is busy using the regen brakes at the front). So it follows that if the Prius can so sparingly apply the friction brakes at the rear, then in a conventional car (as trade off in the design) the brakes are generally being applied too much (but causes less rear brake failures?)?

The Prius friction brakes apply similar proportions of braking to the front and rear as any other car.

As you apply the brakes on a vehicle, the weight of the vehicle transfers forwards, i.e. to the front, therefore more brake force is applied to the front wheels than the back, IIRC 70% front 30% rear. This is usually controlled by a compensation valve in the rear braking system that lessens the rear brake force. This valve is connected to the rear suspension (via a sprung linkage) so that when the rear of the car is loaded up (and the suspension drops) the valve allows more braking to the rear than it would if unloaded.

I suspect the Prius and some more technical systems on other cars, use the ABS/TC ECU to allocate the brake force percentages to the front and rear (and to each individual wheel) and probably doesn't apply as much to the rear sometimes, as it's not needed. Remember, if it applies too much, the rear end will lock up and as said above, will overtake you in some circumstances.

[timberwolf]

Thanks, very interesting.

I had forgotten about the skid control driver aids, such as ABS and VSC. If as with many engineering design decisions there is a compromise or trade off - it may suggest for example that to maximize the regenerative braking, the Toyota engineers have reduced the rear friction braking and consequently have increased the failure rate on the rear brakes. One of the upsides, as Grumpy points out, is that the brake pads have a longer life.

[Grumpy Cabbie]

To confuse matters further it is said that the European Prius front discs and pads are softer than the US/Jap versions as Europeans prefer more 'bite' to their brakes. As such our front brakes will wear out quicker than our friends across the pond, though still not as quickly as traditional cars. It is said that they do not need to replace their front discs or pads within 100k miles - even for taxi use!?!?

But I guess the trade off here are dull, lifeless brakes. Perhaps if we had their harder front brakes we'd end up with more corrosion issues there also?

I could be wrong - and aften am :)

[Madasafish]

To confuse matters further it is said that the European Prius front discs and pads are softer than the US/Jap versions as Europeans prefer more 'bite' to their brakes. As such our front brakes will wear out quicker than our friends across the pond, though still not as quickly as traditional cars. It is said that they do not need to replace their front discs or pads within 100k miles - even for taxi use!?!?

I average between 60-100k per set of brake pads - over some 30 years...It's all down to driving style.. (and I don't hang about much)

[timberwolf]

To confuse matters further it is said that the European Prius front discs and pads are softer than the US/Jap versions as Europeans prefer more 'bite' to their brakes. As such our front brakes will wear out quicker than our friends across the pond, though still not as quickly as traditional cars. It is said that they do not need to replace their front discs or pads within 100k miles - even for taxi use!?!?

But I guess the trade off here are dull, lifeless brakes. Perhaps if we had their harder front brakes we'd end up with more corrosion issues there also?

I could be wrong - and aften am :)

I'd heard about the suspension differences between the USA and Europe at least where the Gen 2 was concerned, I hadn't heard of the braking preference but it sounds reasonable. Changing the spec of the brake pads, might have meant they also changed the spec of the discs?

I vaguely recall reading somewhere about a difference in thickness of the brake discs for the Gen 2 between the USA and UK, but I had perhaps wrongly assumed it was because the USA Gen 2 Prius still had drum brakes on the rear.

Madasafish: Was that in cars with automatic gears? If not then you are not comparing like for like.

[Grumpy Cabbie]

I'd heard about the suspension differences between the USA and Europe at least where the Gen 2 was concerned, I hadn't heard of the braking preference but it sounds reasonable. Changing the spec of the brake pads, might have meant they also changed the spec of the discs?

I vaguely recall reading somewhere about a difference in thickness of the brake discs for the Gen 2 between the USA and UK, but I had perhaps wrongly assumed it was because the USA Gen 2 Prius still had drum brakes on the rear.

Madasafish: Was that in cars with automatic gears? If not then you are not comparing like for like.

The US gen 2 has drums at the rear.

The European versions have different discs AND pads at the front.

Madasafish - It's difficult to compare different cars, tranmissions and uses for brake wear. I do 30k miles around town but I'm sure our long distance driver (North Yorks to Heathrow etc) will probably get 100k miles of pad life out of his automatic car but 15-20k round town. You're comparing apples to oranges.

[johalareewi]

Just had my 60K service and the rear pads are 100% worn (according to dealer).

update:

Have got car back and the 'worn' rear pads.

One of them is pretty much 100% worn.

The other 3 are almost unused.

[timberwolf]

... and what underlying fault in the braking system had caused the one brake disc pad to wear?

[wass]

TBH I would have expected the dealer to offer an explanation and solution for the unexpected wear of just one of the pads. My own suspicion would be either a seized slider or a sticky caliper piston on just one of the rear calipers due to damaged dust seals. I would have regarded the replacement of all the pads to be just part of the remedy since something will have caused the additional wear to just one pad.

[johalareewi]

I will have to quiz the service manager later because I don't take the car to the dealer, their driver picks it up and drops it back. The service dept's messenger just read out what was on the check sheet from the workshop (rear pads worn 100%) and I didn't see the pads until the driver had dropped the car back after hours. As you say, there has to be an underlying problem causing one pad to wear excessively and if it hasn't been fixed, the new pads will go the same way.

[johalareewi]

Service dept say they inspected the braking mechanism once the pads were off to see what was causing the uneven wear. They found the inside slider was stuck forcing the pad to rub on the disk. This has been also been fixed so the new pads should be OK.

[MazEEW]

My Prius is heading for a service at 124k, wonder whether to give in and change the brakes, last year at 114k the mechanics said they were 70% worn. When asked how far he thought they had been he guessed 30-40,000 miles!

[Hicardo]

Maz..I guess the question is, is it unsafe, or, will it run safely to the next scheduled service? If the answer is no, or there is any doubt, replace them now.

[johalareewi]

Service dept say they inspected the braking mechanism once the pads were off to see what was causing the uneven wear. They found the inside slider was stuck forcing the pad to rub on the disk. This has been also been fixed so the new pads should be OK.

Update:

Car is in for 90,000 mile service.

Dealer has just called to say the (outer) rear brake pads are down to the metal on both wheels.

The reason given for them wearing down so quickly is that the rear discs get too hot and wear the pads down faster.

[timberwolf]

Mmmm. Conclusion underlying fault not fixed during last pad change.

[kithmo]

Service dept say they inspected the braking mechanism once the pads were off to see what was causing the uneven wear. They found the inside slider was stuck forcing the pad to rub on the disk. This has been also been fixed so the new pads should be OK.

Update:

Car is in for 90,000 mile service.

Dealer has just called to say the (outer) rear brake pads are down to the metal on both wheels.

The reason given for them wearing down so quickly is that the rear discs get too hot and wear the pads down faster.

What are they made out of, plastic ?

[johalareewi]

What are they made out of, plastic ?

Butter maybe?

[APS]

Coming back to the original question about binding rear breaks after washing the car. This is normal.

Many Toyota's (and other cars) have the parking break as a drum break inside the rear discs. When you wash the car you will get some water or moist in the parking drum breaks. If you then park your car over night with the parking break applied, they will be stuck the next morning - and can stick quite hard if there was a lot of moist in the drums when you parked it.

Therefore, never park the car after washing it without driving it first. Go for a short drive and make sure you apply the breaks hard a couple of times. You can also apply the hand break momentarily to dry out the drums (don't know if this is recommendable with a foot operated break like the Prius).

About rear breaks - in general they don't get enough 'exercise'. Particularly on a prius which if often driven economically. They fare best when the car is driven on longer journeys with frequent use. The real killer for breaks is winter. The combination of salty slush and very light use of the breaks due to slippery road conditions means the break never warm up properly and the rust will gain foothold on discs, calliper pistons and guide pins. This is when you end up with one totally worn out pad and unevenly worn pads and reduced friction surface on the discs.

Summa samarium - make sure you use your breaks particularly if the weather is wet or you live in a cold and wet climate.

[johalareewi]

Brakes

[Guest]

^^^^^ what he said.