Rear brake pads "knocking"

[Owainsdad]

Have an issue with our 2005 VRS where there is a knocking noise coming from the rear of the car when going over bumps and drain covers, which goes away if the brakes are applied.  I nipped the handbrake cable up tight and it went away, but obviously the handbrake was binding so I had to back it off.  When set correctly the noise was less frequent, but still there - going away when I brake.

 

I've not had chance to have a wheel off yet, to investigate, but any ideas gratefully received.  New calipers, discs and pads on the rear nine months ago for the MOT.  Calipers were not OEM, judging by what the garage charged for them.  They do have the handbrake return springs fitted correctly, though.  Wheel nuts all tight, no wheel bearing droney noises, just a loud, annoying clanking on any bumps, etc.

 

Cheers

 

Dean

[HeavyMetalRich]

Check your wheel bolts first. The ones on my Fabia worked themselves loose having checked them with a torque wrench a couple of times, the wheel didn't move much when I jacked the car up but it was obviously enough to make a noise similar to what you described. It also seemed to disappear when I applied the brakes.

 

If you rule out wheel nuts, check that your rear shock absorber top mount casings are still whole. You might have to remove the wheel arch liners to see them fully.

 

Another possibility might be the disc backing plates if they're still present. They might rattle if bent or about to drop off.

 

Then if those are ok, do your pads have the correct shims and do they still have the backing pads, that acts as a shock absorber of sorts, between the caliper and piston.

 

That last thing I can think of to check if all of the above is fine would be slide pins, check that they're not overly worn and causing too much movement.

 

I hope this helps and gives you a few ideas.

[Owainsdad]

Thanks very much Rich.  That gives me a good load of things to work through! 👍

[Carlston]

The rear brakes on the Fabia MK1 VRS are a TRW brake system, so it makes sense to use TRW rear brake pads. These shouldn't be noisy as they will have had a lot of R&D.

 

TRW GDB1330 rear brake pads (for Fabia MK1 VRS)

 

https://www.autodoc.co.uk/trw/2191943

 

Edited 16 August, 2023 by Carlston

[Owainsdad]

On 15/08/2023 at 13:46, HeavyMetalRich said:

Check your wheel bolts first. The ones on my Fabia worked themselves loose having checked them with a torque wrench a couple of times, the wheel didn't move much when I jacked the car up but it was obviously enough to make a noise similar to what you described. It also seemed to disappear when I applied the brakes.

 

If you rule out wheel nuts, check that your rear shock absorber top mount casings are still whole. You might have to remove the wheel arch liners to see them fully.

 

Another possibility might be the disc backing plates if they're still present. They might rattle if bent or about to drop off.

 

Then if those are ok, do your pads have the correct shims and do they still have the backing pads, that acts as a shock absorber of sorts, between the caliper and piston.

 

That last thing I can think of to check if all of the above is fine would be slide pins, check that they're not overly worn and causing too much movement.

 

I hope this helps and gives you a few ideas.

 

Just to update on this one, in case it helps someone else.

 

Worked through Rich's comprehensive list (thanks again for this Rich) and drew a complete blank.  However, whilst it was up in the air with the wheels off, I could see the pads were back slightly from the discs.  I got my lad to press the brake pedal and could see that the pistons moved correctly, and pushed the pads on to the discs correctly, but when he released the pedal the pads were pulling back again, a few mm from the discs.

 

This rang alarm bells, as they should stay put, and could only be pulling back if there was a "vacuum" in the master-cylinder reservoir.

 

I took the cap off the reservoir, and got the lad to put the brake pedal to the floor and when he released it the pads stayed where they were and didn't pull back.

 

I've replaced the cap now, and driven the car some thirty-odd miles and the knocking is completely gone, and the pedal also feels better, as the first part of the pedal travel is not now being used up sending the rear pads back into contact with the discs.

 

I can only assume that as the new pads and discs (about nine months ago) have worn that the air above the brake fluid has not been replaced, causing a vacuum in the reservoir/master-cylinder, that pulls the pistons back in.  I doubt it was down to anything the garage did when they changed the calipers, pads and discs for me, as they would have had to have bled it all afterwards - which would have necessitated taking the cap off to refill the fluid.  So I can only assume that the cap should be vented, in order to allow air in to account for wear, but mine must be blocked or faulty in some way.

 

Anyway, I'll see if the problem returns as the pads wear further.....

 

[sepulchrave]

Yes, the cap should be vented, well spotted, a good fix for a very rare problem.