Recently discovered our XJs don't have a brake pad wear sensor when I heard the horrible metal on metal sound coming from my rear brakes. It came at a bad time so I paid the "premium" and bought the discs, pads and rented the tool from my local AutoZone and replaced both rear brakes.

After driving for a while I noticed I was getting a bit of vibration when braking from the rear. Not sure if its through the pedal or the whole car.

I checked the discs and I can see that the right hand side is wearing nicely whilst the left side is not wearing at all. I can still see the crosshatch marks from the new disc.

So yesterday I took the whole thing apart on the left side and went through it all, double cleaned everything again and refit it. I even made sure the piston was moving by slowly pressing the brake pedal without the pads and then rewinding the piston back. Obviously not going too far as to pop the piston out of course.

Anyways so everything is bolted back up but still the same problem. There is no wear occurring on the left side and I get a vibration when braking.

To me this feels like some sort of ABS or brake distribution malfunction. Its not sending the appropriate amount of brake pressure to the left rear.

No error/warning lights.

Any ideas?

 

Did you ensure that the caliper 'slider' pins are free while things were apart? Did you recalibrate the EPB and does it work properly, independently of the foot brake?

 

Hi Mikey.

Yes of course I cleaned the pins thoroughly and lubricated them. Everything moves nicely. Recalibrate the EPB? I don't think I've done that. After replacing the brakes I pumped the pedal about 10 times till it there was absolutely no movement. I held the EPB off while starting the car and then I put it ON and OFF about 10 times. Everything sounded good, no errors messages. Not sure how to test the EPB to see if it worked independently of the foot brake.

 

With the transmission in drive, let the car coast along at low speed. Instead of using the foot brake, Apply the EPB by lifting the paddle several times. The parking brake should bring the car to a surprisingly firm stop. Release the EPB by pushing down on the paddle and the car should resume moving again. Any noticeable difference from side to side would indicate a seized EPB mechanism.

 

There is probably air in the line. You need to bleed the brakes.

 

Thanks for the suggestions.

I don't think the EPB would affect the brake force applied by the footbrake but I suppose testing it doesn't hurt.

As for fluid, its probably due for a change anyway so I'll try do it this weekend but then again, how would air have got in the lines? The pedal feels rock solid.

 

If the caliper EPB mechanism is seized the foot brake will not apply normal pressure to the pads.

 

While you're bleeding the brakes, you could easily do a visual inspection. If the system is pressurized should be able to see the caliper clamp onto the rotor.

 

That's interesting. Care to elaborate?

As I said in my opening post, the piston moves freely when the foot brake is applied. Its just not given sufficient hydraulic pressure to stop the moving wheel when on the road.

I am thinking of swapping the left and right wheel lines on the distribution block to see if the problem moves to the other wheel. That should eliminate whether its a caliper problem or distribution block problem.

 

Forgot about that. Thanks for the reminder.

You might be able to see it move, but can you see the caliper clamp and deflect due to high pressure? If you were close by, I'd have you come over so I can hook up a pressure gauge I use for testing line pressures at the calipers on my race cars. The pressure at both rear calipers should be the same if there is no air in the line. I know you said that the pedal feels solid but it's odd that this one caliper isn't clamping onto the pads.

 

Hey V12:

The parking brake mechanism in the rear calipers can drive you nuts. I've experienced the same issue as you're having with Audi (A8). A much better design uses mini brake shoes/drum for a dedicated parking brake (eg, Porsche; BMW M cars). Problem is that its hard to get enough (manual lever) pad pressure when you have relatively large pads

As Zazzy suggests, bleed the caliper first (sometimes an air bubble forms in the caliper when you're moving it about)

If no joy, rebuild the rear calipers, making sure that the caliper piston PB winders are completely free. Or, best, replace the calipers (use OE)

BTW, depress the brake pedal (with a stick) and keep it that way before cracking open brake lines. This keeps fluid from draining out of the ABS system (v hard to bleed) all over you and the floor

Hope this helps!

 

The EPB mechanism also compensates for pad wear. If the mechanism is seized (as mine was) the pads will soon make insufficient contact with the rotor to provide adequate braking. That's why I suggested the simple test above.

 

My EPB caliper is a completely separate caliper. The hydraulics on the rear brake caliper are independent of the parking brake. Are you suggesting that the electronics for the parking brake will prevent pressure to a rear caliper?

 

Flushed the brake fluid. Took almost 1 quart to get it all clean. Nasty stuff. See photo.

But alas no difference. There is nothing happening on the left side. The pressure is just sufficient to take the light surface rust off but nothing more than that.

Pulling the EPB while doing about 20 mph, I could get the right hand side to lock up sometimes. But not always and I was too chicken to try go any faster. Does ABS intervene with the EPB?

To complicate matters further, while driving or when testing the EPB, I couldn't feel any obvious pull in either direction. Car brakes straight.

I did do the EPB recalibration a few days ago as a doesn't-hurt-to-try thing but didn't make a difference.

Like a stupid mechanic I'll replace the left caliper as a next step and see what happens. Cheaper than taking it to the dealer for a diagnosis.

 

Both sides should lock up solidly by the third of fourth pull of the paddle.