Author Topic: Diagonal dragging brakes (rear DS / front PS)  (Read 143 times)

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Offline Moonshine

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Diagonal dragging brakes (rear DS / front PS)
« on: July 08, 2019, 12:38:53 pm »
I am having issues with my Mazda 6 brakes and can't quite get my head around the issue.

I was having issues with the rear DS brake, which looked like a stiff slider due to inconsistent pad wear, with the outer pad nearly worn out.

The caliper wasn't looking to good either, so I have replaced the caliper and the caliper carrier, bled the brakes and went for a test drive.

All seemed fine with firm brakes, however when i got back i felt all discs and both the rear DS / front PS were far far hotter than the other discs, indicating that the pads on dragging on these diagonal brakes (I don't know if the front PS brake dragging was a pre-existing issue)

I took the wheels off these hot brakes and checked the sliders (all fine and free) and the caliper retraction, and the front caliper was very stiff, but went in eventually with a G-clamp. The rear goes (new caliper) in stiffly while being twisted (brake fluid reservoir cap on).

It seems that there is an issue with how these diagonal brakes retract, and the brake system allows them to free up a little to release the brake, but not enough that they don't drag, causing the disc to get too hot.

I have read that car brakes work on a diagonal redundancy system, with the two diagonals working on different circuits with the master cylinder having two chambers / pistons. Could it be an issue local to the rear DS / front PS circuit, or a fault with the master cylinder for that circuit.

The other two brakes seem to work fine, with not excessive heat in the discs.

Any ideas on fault finding?


Offline Moonshine

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Re: Diagonal dragging brakes (rear DS / front PS)
« Reply #1 on: July 12, 2019, 09:14:33 am »
As an update to this, i checked out the front caliper and the piston was in a bad way and dirt build up looks like the culprit, causing it not to be able to retract fully and drag.

I overhauled the caliper and piston and re-attached and bled, seems to have solved the issue and the front is no longer dragging.

I also re-bled the rear and 'seems' to have solved the issue.

Offline apav

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Re: Diagonal dragging brakes (rear DS / front PS)
« Reply #2 on: July 12, 2019, 10:33:41 am »
It is usually like that because they are exposed to similar conditions.

So the other side may give up soon but you are lucky you can work on them.

If you have the time, you may save trouble by giving the time to strip all of them, clean them and put them back.

I have seen people with newish cars, e.g. just after the warranty expired, when they service the car to a small but passionate garage, they strip the brakes and put them back again.

Unfortunately this is not a standard procedure in any of the service intervals and I think that is why the brakes fail.

Maybe it is a cost effective exercise to have callipers replaced every so often than cleaning the brakes on a regular basis.