Rear Brakes Trouble 2021 Nissan Sentra

[Paperdoc]

Anyone else have this issue? My daughter has a 2021 Nissan Sentra SR sedan, bought new fall of '21. My son has an SV model of the same car, same year, bought used summer of '22. Both cars are used almost exclusively for shorter drives to and from work in our small city in Northern Ontario. Each bought a second set of steel wheels to mount winter tires. So I swap summer / winter tires each fall and spring. In doing that I always inspect brakes, etc. exposed for full view. On both cars right from the start (including the first spring of '22 on my daughter' car) I noted in their service log books that on BOTH rear wheels the disk brake rotors showed only mild wear and some areas of apparent no wear at all. In subsequent changes the low-wear areas got worse and began to show light pitting and rust. Although I could see clearly only the outboard sides of each rotor, I recently had opportunity to see the inboard sides of the ones from my daughter's car, and they look exactly the same. The front brake systems on both cars look perfectly normal with slow wear of rotors and pads, clean surfaces and no signs of rust or pitting.

To me, this says that the rear brake pads are not clamping onto the rotors to wear them down at all, or just barely. At first I though that means the design somehow does not balance the use of front versus rear brakes and the rears are doing significantly less work. But as the pattern got worse I began to worry that the rear brake units were seized. That is, maybe the Caliper unit pistons were not moving under hydraulic pressure, so the brakes were doing nothing. That would account for no contact of pads with rotors on either side. OR the mounting pins that caliper units slide on for centring were seized up and the calipers units simply cannot slide. But that should result in inboard pads still pushing against the inboard side of the rotor, while the outboard side would not. That this should occur on four out of four units raised my suspicions.

Yesterday my daughter took her car in for several several routine service items that included tire rotation. (Not needed since I do that when swapping winter / summer wheels, but that's part of the dealers' package.) They diagnosed bad rust damage and pitting to the rotors on both rear wheels and resultant damaged pads and replaced them all. But they say the caliper unit pistons are moving freely and need no servicing. I questioned them closely on this and they showed me the rotors they had removed.Those are exactly as I had observed. The service manager seemed unimpressed with my reasoning about how such lack of wear could happen, and said it is just rusting of the rotor metal from infrequent use of the car for longer drives and low brake unit use.

So, anyone else have experience with the rear brakes of a 2021 Nissan Sentra? Anyone with LOTS of use, versus our low-mileage uses?

 

[Paperdoc]

Last first: I would say both children drive modestly to conservatively - no sudden acceleration and braking. As I said above, most drives for them are short and low speed. My own driving trips are similar but perhaps more frequent than theirs, but I'm more aggressive that they, although I do not believe I'm reckless!

I'm certainly accustomed to uneven wear of pads. But what I usually see if they are removed for examination is that the pads have arced ridges in them that match shallow grooves in the rotor surfaces. Further, in those cases on the ROTORS the entire contact surface is shiny clean from the abrasive action of pads on rotors. That action constantly removes any slow build-up of rust. In this current situation that clearly is not happening, and it is the same on both sides of reach rotor. What I do not know is whether that is because the pads simply are making no contact with the rotors, or because there is light contact only because the system concentrates most braking effort in the front brakes. In either possibility, light or no contact might NOT "scrub" the rotor surfaces constantly enough to prevent rust build-up.

 

[mindless1]

It is probable that it has significant front wheel braking bias, applying far more force to the front wheels.

That does not mean that the back should not be engaging enough to at least scrub the rust off, no matter how conservative the driver is (possibly less so with an OCD hypermiler who literally only drives when there are no other vehicles around so they hardly ever have to apply the brakes.

If the caliper slide pins are near seized, that does not mean that the back will make same contact. The whole caliper floats and insufficient movement will make both the front and rear pads, lack clamping pressure.

I am not, "necessarily" suggesting this is the problem, but something was a problem. The shop seems unwilling to investigate and do more than sell you new pads and rotors, so I would no longer let that shop service the brakes... except see below.

There is no substitute for a hands-on inspection by someone who cares to get it right. What is most likely is that excessive corrosion on the caliper slide rail, has caused the pad ears to get into a bind and not slide freely/evenly. 2nd most common would be the caliper slider pins except that with these being relatively newish vehicles, unless Nissan's QC is horrific, they should not have dried out lube "yet", but certainly if I were taking a comprehensive look at it, the caliper would be removed and the slide pins checked for both smooth movement, and pulled out to check for sufficient lube on them - going ahead and relubing them while out, or replacing if the factory lube is dried out and they are rust pitted - but again at this young age, they shouldn't be that bad yet while the caliper slide rails, being exposed to the elements can rust much faster.

If it is the caliper slide rails, remove the shims, clean off any grease residue (if even present), true up the slider channels with a flat file or wire wheel or sandpaper if you're a glutton for exercise, apply silicone paste to deter rust, and slap the stainless shims back in - or replace the shims if they are now deformed.

I don't think the pads got damaged form the rotors, not the rust nor the pitting, but rather from uneven contact and this caused the rotors to wear unevenly. Pitting won't hurt anything and given new pads with normal contact, in most cases that would remove the need for new rotors unless the wear on them was too uneven which is possible.

It is possible that when the shop replaced the pads, they rectified the problem, but I would check it myself to be sure they didn't just do a pad slap since you didn't state that they found the reason and just made up nonsense about infrequent use and low brake usage. Infrequent use (parked for weeks to months at a time) will cover the entire rotor will rust but that will just feel notchy the moment it is put into drive or reverse and the pads hit that edge of the rust buildup, and will all get scrubbed off evenly if all else is good. It could be that the mechanic doing the work, did it right but the service manager doesn't know squat!

 

[Paperdoc]

They did actually replace the rotors - I saw them in the shop junk barrel, which is how I know what BOTH sides looked like. I did ask particularly whether the "seized" part might be the slide bolts, but thought (mistakenly, you say) in that case the inboard pad still would push out to make good contact with the inboard rotor face. I did not consider whether the pad ears might seize in their small channels because I thought the hydraulic force would overcome that and force them to move. Maybe not.

Well, I cautioned my son who has virtually the same car with the same symptoms NOT to take it to the dealer. I do brake work all the time, so I'll order parts and do the job. Now I will know exactly what to look for, so maybe I can get a better cause determination.

On my 2014 Mazda 3 there have been times when I do not use it for a week or two. On those occasions I have noted the first time I touch the brakes I can hear a light "grinding" sound as the pads start to scrub rust off the rotors. This lasts for several brake uses until they are all cleaned off, then the sound stops. It appears to have no impact on brake performance. So I'm used to rotor rust and how it disappears in normal use.

So far, nobody has said "Oh yeah, this is a common problem with that car". So maybe it is not a design flaw.

 

[Mo007]

Hi Glad to make your acquaintance, my Son have a 2021 Nissan Sentra Sr here in the US. about 53K miles and dealer told me rear brakes in need of change ok. lots of money to do it. Anyway I took a look at it and was surprise. I have been doing brake job on a 96 nissan maxima gxe, honda pilot 2004, Acura TL 09, I never seen rear brake pads worn so unevenly from OEM installation on brand new vehicle. So I think the dealer will be sucking people dry with this issue. I have not review the front as yet. when you have a sec please confirmed torq specification your are using. I don't have the service manual as yet.

 

[Mo007]

How many miles, on the recommended brake change for your daughter car. Do see you the nonsense brake clip they use. I wonder if that is causing the brake pad from sitting flush?

 

[Paperdoc]

I have not yet actually done a rear brake replacement for my son's '21 Sentra. Search on the internet says the bolts that mount the caliper BRACKET to the steering knuckle at 62 Ft-lb., and the smaller bolts that the caliper unit slides on at 20 ft-lb. Do NOT use the higher spec for the slide bolts - they will break!

I'll get a better review of the two cars' rear brakes this spring when I switch back to summer tires. Last fall when I put on their winter wheels the brakes on my daughter's car (which HAD been replaced by the dealer) showed very little rust on rotors, while the ones on my son's car (not replace yet) still show the same substantial rusting.