The 'E' Type Forum

Has anyone experienced this issue that I believe is associated with the rear brakes on my S3?

After I have been driving the car for approx 20 mins or so, I get a distinct knocking sound from the rear of the car when I brake gently. It doesn't occur when I initially take the car out and brake. The knocking is commensurate with speed.

On a scale of 1 to 10, where 1 is just touching the brake pedal and 10 is an emergency stop, it occurs from 1 to 6, so for example, when pulling up at traffic lights.

If I increase the braking whilst the knocking occurs, then the noise goes away.

The IRS came out a little while ago and I inspected the UJs, the discs and pads, and all seemed fine.

I put some new pads in yesterday but this has made no difference.

A fellow E Type owner has suggested that it is associated with the brake pad just making contact with the disc resulting in some free play and movement of the pad against the caliber.

If anyone has experienced this or has any ideas or remedies, then I'd be grateful to hear. Thanks.

I don't know if it is the same thing, but I had problems with brake squealing which was diagnosed as the pads not retracting fully from the disc.
The fix to getting the pads to fully retract was to fit some adhesive behind the pad to make it stick to the piston.
this may help your problem if the pads are not fully retracting.

check your calliper bolts are tight. I had this problem on the front of the car and found the bolts were loose which allowed the calliper to move slightly.

Dave

Pretty sure the caliper bolts are tight, as I had the IRS out a few months ago and I would have checked this. However......just a double check won't hurt.

Perhaps I will have to resort to sticky stuff on the pads.

Thanks for your advice.

Sounds like the discs may need skimming. Were they machined or replaced?

I have no idea when the discs were last attended to. They weren't in tip top condition when I had the IRS off the car, but certainly good IMO for a couple of seasons driving.
Having said this, I presume that a disc would only have to be fractionally distorted to cause knocking, and perhaps this would be difficult to assess by my amateur eye.

Dear Marcus,

If the discs are warped, then they'll tend to push the pads back and this will displace brake fluid. You ought to be able to feel the push back against your right foot.

If it is speed related then I'd look to make sure that anything along the drivetrain is fastened, e.g. the bolts between the differential and the propshaft (since you have undone those recently), half shafts, u-joints, etc as the symptoms you describe imply that load changes with change of speed leave something loose.

You can rule out the main pads by braking (gently) using the handbrake from slow speed. Even if a disc were warped, it could still be bolted down and not be loose - that means it won't knock.

You can easily check the brakes during driving as you have a s3. Peel back the rear carpets, open the inspectionn hatches and invite Mrs Marcus on a test drive. When braking, you (she) can thus watch the calipers, pads et al through the inspection hatches and maybe even rest a finger or dowel on the caliper body to feel if that is where the vibration comes from.
This might be easier done with the parcel shelf removed and a couple of rulers to prop up the carpets.

kind regards
Marek

Hi Marek,

Thank you for your advice.

When the IRS was out I did have a good look at the UJs and half shafts etc , and I've also checked the prop shaft bolts, as this was my initial thought.

It only knocks after approx 20 mins of driving/ braking, which I guess points to the disc/s warming up and the tolerance altering.

I don't feel the knocking through the brake pedal at all.

I will perhaps try braking with the handbrake only next time I go out, to see if this makes a difference, as you suggest.

As I mentioned earlier, if I brake firmly every time, then I can avoid the knocking. It only occurs during light braking.

Your suggestion about getting an observer to get up close to the exposed brake unit to watch for anything untoward whilst driving is a good idea. I'll certainly invite the Memsahib along for this task. After all, I think if she wishes to continue to enjoy days out in this car, she ought to share in the maintenance!

Thanks again.

Hi Marcus....once u have your car warmed up and u have the knocking get the car home and the rear end jacked up and run the car in gear, then use your brakes, u will then be in a stationary position to easily locate the source,....it does feel a bit unnerving to have the car running whilst up in the air, so just think over how u will do it.....u will need a helper to do the braking whilst u hunt the fault....Steve

Another welcome piece of advice. Thanks Steve.