# Rear brakes not engaging



## Guap0_ (Dec 2, 2017)

I hope that you didn't step on the brake too hard without the pads & the discs installed. That could have cocked & locked the pistons preventing them from returning. Put it all back together except for the tires. If it's a rear wheel drive car, jack up both rear wheels & put it in gear. Step on the brake. See if the wheels stop tuning. If it front wheel drive, leave it in neutral, spin the wheels by hand. Have someone step on the brake. See if it prevents you from turning the wheels.


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## maholmes9 (Oct 10, 2018)

Thanks for your reply! I stepped lightly on the brakes, just to see if they would move at all (this was after realizing they were not engaging, and attempting to see what the problem was). I thought maybe the calipers were seized for some reason, but this doesn't seem like the case, as they were able to move. I then turned them in enough to get the pads and rotors back on, but still not engaging. The car is an AWD (Quattro).


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## Nealtw (Jun 22, 2017)

Are the pistons rusted in place. Do you have hydraulic pressure at the back wheels. 
Are you getting pressure from the master.


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## Guap0_ (Dec 2, 2017)

What test are you using to conclude that the brakes are not engaging? Did you try the test that I suggested? Spin the rear wheels by hand (in neutral) & have someone step on the brake.


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## maholmes9 (Oct 10, 2018)

No, the pistons do not appear to be rusted in place.They look to be in good condition. I'm not sure if I'm getting hydraulic pressure at the back wheels. Is there a way to check this?


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## maholmes9 (Oct 10, 2018)

No, I will have to try that later. I believe the brakes are not engaging because there is no indication that the pads have come in contact with the rotors, as the rotors have rust on the face, whereas the fronts look fine after driving.


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## Guap0_ (Dec 2, 2017)

That's not a good test. The weight shifts to the front when the brakes are activated so that would remove the rust sooner than the rears.


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## maholmes9 (Oct 10, 2018)

Yeah, that makes sense, but the rust has been more and more. It's not like the amount from just sitting for a day or two (or 3 or 4), but quite a bit more than that. My mechanic mentioned it also when I brought it in for something else, but I didn't bring it back for a diagnosis. I'm looking to sell the car and would rather put only a minimal amount of money into it. I do only typically drive it about 4 miles a day in the city, but some times more than this and the rust stays there. 

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## Nealtw (Jun 22, 2017)

maholmes9 said:


> No, the pistons do not appear to be rusted in place.They look to be in good condition. I'm not sure if I'm getting hydraulic pressure at the back wheels. Is there a way to check this?


 Bleed the breaks???


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## maholmes9 (Oct 10, 2018)

Nealtw said:


> Bleed the breaks???


Yeah, that's what I was thinking about doing. I noted that near the end of my initial question, but didn't want to "spin my wheels." I was wondering if someone had a different/better suggestion. Thanks. 

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## Nealtw (Jun 22, 2017)

maholmes9 said:


> Yeah, that's what I was thinking about doing. I noted that near the end of my initial question, but didn't want to "spin my wheels." I was wondering if someone had a different/better suggestion. Thanks.
> 
> Sent from my SM-N950U using Tapatalk


 Have you checked the fluid level in the master?


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## maholmes9 (Oct 10, 2018)

Nealtw said:


> Have you checked the fluid level in the master?


Yes, I checked it and it looked full. I will check it again to make sure. 

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## Guap0_ (Dec 2, 2017)

I wouldn't bleed the brakes. I don't think that there is air in the system.


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## Nealtw (Jun 22, 2017)

Guap0_ said:


> I wouldn't bleed the brakes. I don't think that there is air in the system.


 Well, what would you consider a hint there might be air in the system. :wink2:


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## Guap0_ (Dec 2, 2017)

The pedal would be spongy & possibly low. I don't know why he doesn't want to do the test that I suggested. It doesn't make any sense to me.


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## maholmes9 (Oct 10, 2018)

Guap0_ said:


> The pedal would be spongy & possibly low. I don't know why he doesn't want to do the test that I suggested. It doesn't make any sense to me.


I will do what you suggested, I just can't do it right now. What do you think I should check after if it does move or doesn't move? 

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## Nealtw (Jun 22, 2017)

Guap0_ said:


> The pedal would be spongy & possibly low. I don't know why he doesn't want to do the test that I suggested. It doesn't make any sense to me.


 With twin master and the front is working may not feel spongy and may not even pump up depending on the amount of air or if it had a leak.


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## iamrfixit (Jan 30, 2011)

Most likely you have a broken wire to the speed sensor, could be further up the harness or in a plug somewhere. Check especially carefully where the wire has to move or flex to follow the suspension. Can be hard to find a broken wire, I've found wires broken and not making connection inside the insulation with little or no visual sign when simply looking at the wire. You have to test, visual inspection won't always tell the story. 

Not only must the piston move, but the caliper has to slide. Your piston may be contacting the back side of the rotor but not pulling the outboard pad over against the rotor. You will probably have either slide pins or slide bushings. Some older vehicles rely on the caliper to actually slide against the caliper mount. You'll need to remove, clean up and grease the sliding mechanism with caliper grease.

If you have air in the system the pedal will almost always be soft or seem to go too far down to the floor, even if it's only in half the system. Air is easily compressed, unlike brake fluid so even a small amount of air will greatly affect your brake system's ability to build up pressure. Pumping the brake will usually push more fluid into the lines which further compresses the air and allows the pedal to pump up a bit, but it will not stay, the air that's present in the line will push the extra fluid back into the master cylinder as soon as you take your foot off the brake.


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## maholmes9 (Oct 10, 2018)

iamrfixit said:


> Most likely you have a broken wire to the speed sensor, could be further up the harness or in a plug somewhere. Check especially carefully where the wire has to move or flex to follow the suspension. Can be hard to find a broken wire, I've found wires broken and not making connection inside the insulation with little or no visual sign when simply looking at the wire. You have to test, visual inspection won't always tell the story.
> 
> Not only must the piston move, but the caliper has to slide. Your piston may be contacting the back side of the rotor but not pulling the outboard pad over against the rotor. You will probably have either slide pins or slide bushings. Some older vehicles rely on the caliper to actually slide against the caliper mount. You'll need to remove, clean up and grease the sliding mechanism with caliper grease.
> 
> If you have air in the system the pedal will almost always be soft or seem to go too far down to the floor, even if it's only in half the system. Air is easily compressed, unlike brake fluid so even a small amount of air will greatly affect your brake system's ability to build up pressure. Pumping the brake will usually push more fluid into the lines which further compresses the air and allows the pedal to pump up a bit, but it will not stay, the air that's present in the line will push the extra fluid back into the master cylinder as soon as you take your foot off the brake.


Would a broken wire to speed sensor throw a code? I do get the code for a short or plus to ground for front left wheel speed sensor, but I assume this would have nothing to do with the rear brakes. The car calipers do have slide pins, which, when I tested the pistons, I removed, cleaned, greased, and check that they moved freely, which they did. 

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## Guap0_ (Dec 2, 2017)

> I will do what you suggested, I just can't do it right now. What do you think I should check after if it does move or doesn't move?


I think that the rear brakes will stop the wheels from spinning no matter if you spin them by hand or if you run the car on a lift. If it works, just clean rust off the discs. If it doesn't work, something is stopping the brake fluid from reaching the calipers.


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## Gregsoldtruck79 (Dec 21, 2017)

Those ABS controls can get real complex as to the way they operate for each model vehicle as they get newer in model. They have a lot of fluid controls built in to them to accomplish their task of preventing total wheel lockup during panic braking. While using wheel speed sensors that are communicating with transmission speed sensors etc. Serious type head scratching stuff for sure. 

So with the ABS code lights being on and no fluid pressure to the back calipers, my *non -pro opinion* is the ABS control unit has a internal failure. JMO though


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## Guap0_ (Dec 2, 2017)

The ABS on my vehicle is so bad, I have to pull the fuse every winter. It will not let me stop if there is even an inch of snow on the ground. He can pull the fuse to test the brakes.


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## SeniorSitizen (Sep 10, 2012)

Guap0_ said:


> The ABS on my vehicle is so bad, I have to pull the fuse every winter. It will not let me stop if there is even an inch of snow on the ground. He can pull the fuse to test the brakes.


That's to keep your vehicle from going through a red light intersection rear bumper first.


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## Guap0_ (Dec 2, 2017)

It's a worthless invention if I can't stop at all.


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## SeniorSitizen (Sep 10, 2012)

Guap0_ said:


> It's a worthless invention if I can't stop at all.


That's a similar statement of all non believers, but I prefer going through the intersection front bumper first. Some prefer rear bumper first, just not this ole man.:biggrin2:


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## Dave Sal (Dec 20, 2012)

In my opinion, non-functioning rear brakes are usually a seized caliper, a bad flexible brake hose, or something wrong with the master cylinder. Open the bleed screw like someone else mentioned, have someone gently pump the brakes and see if brake fluid comes out. If not, change the flexible brake hose going to the caliper, then bleed the system.


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## Guap0_ (Dec 2, 2017)

> That's a similar statement of all non believers, but I prefer going through the intersection front bumper first.


I prefer to stop for a stop sign. ABS prevents me from doing that when there is snow on the ground. I don't know about your bumper stories.


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## SeniorSitizen (Sep 10, 2012)

Guap0_ said:


> I prefer to stop for a stop sign. ABS prevents me from doing that when there is snow on the ground. I don't know about your bumper stories.


 I'll make this short so not to hijack the the O P subject more.
The ABS doesn't prevent you from stopping, the snow does. It's difficult to convince anyone that tires that are sliding ( locked wheels ) whether on snow or concrete, have less traction than tires that are rolling. It's been proven so just believe it. And it's also difficult to convince some that wheels that have the least traction ( locked rear wheels ) usually take the lead, hence - _rear bumper through intersection first._


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## lenaitch (Feb 10, 2014)

Certainly not a pro (or even a half-decent backyard mechanic) but my understanding is that a failed ABS system is designed to not impair brake operation - you just won't have ABS. One of the major motorcycle manufacturers is currently dealing with allegations that their ABS system will, in fact, cause the brakes to malfunction if it fails.


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## Nealtw (Jun 22, 2017)

Talking about the more expensive stuff is insane before you know that when you step on the break the fluid is moving to the wheels. You check that by bleeding, five minute and free.


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## Bigplanz (Apr 10, 2009)

lenaitch said:


> Certainly not a pro (or even a half-decent backyard mechanic) but my understanding is that a failed ABS system is designed to not impair brake operation - you just won't have ABS. One of the major motorcycle manufacturers is currently dealing with allegations that their ABS system will, in fact, cause the brakes to malfunction if it fails.


ABS can fail and the regular brakes still work fine. We drove the Windstar all over the state with an ABS light on. I finally replaced a bad wheel sensor and two weeks later had to slam on the brakes. ABS started chattering away, doing its thing.


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## Gregsoldtruck79 (Dec 21, 2017)

I know the ABS is not supposed to cause normal braking failures but I was just wondering though. 

Doesn't an ABS control unit work on the interference of hydraulic pressure to each wheel cylinder, as the wheel speed sensor sends the ABS control signals when hard braking ? 

So to me, if there is a portal bleed off or control seat inside that ABS master control unit that malfunctions permanently, could it not cause normal braking operation issues … as in the rear brakes for the OP ?


(As Neal and others have stated, the OP FIRST needs to verify that brake fluid with NO air in it, being present at the rear brake cylinder's under pedal pressure via the wheel cylinder's bleeder valve )


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## Guap0_ (Dec 2, 2017)

senior wrote:


> The ABS doesn't prevent you from stopping, the snow does.


When I disable the ABS, all works well. You come here when it snows & drive my vehicle. Bring extra insurance.


Neal wrote:


> Talking about the more expensive stuff is insane before you know that when you step on the break the fluid is moving to the wheels. You check that by bleeding, five minute and free.


With all that rust that the OP was describing, it won't be free as soon as the bleeder screw breaks. I'm sure that all of you have experience with that.


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## Brainbucket (Mar 30, 2015)

I don't know what vehicle we are talking about. Might have missed it. But I have seen this before. Since OP has said the caliper pistons move freely, then I would start bleeding brakes at the master cylinder first to see if I have pressure. If I do, then I would bleed at the ABS unit to see if I have pressure. I have see a lot of ABS units back feed pressure through the unit because of bad/old fluid and makes the unit faulty. Flushing the brake system is necessary in the modern vehicles of today with ABS according with the maintenance of the manual.:vs_cool:


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## Oso954 (Jun 23, 2012)

> The car is an AWD (Quattro).


Is it Electronic Stabilization Program (ESP) equipped?
If yes, that changes the brake bleeding sequence.
The Audi procedure specifies pressure bleeding at a minimum of 2 Bar.


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