Hot Front Brakes

[BrendanM]

On Saturday I replaced the right front brake disc and pads on my 2.0 i petrol turbo estate auto.
I only managed to do the right hand side due to time and the weather.

When I used it on Monday morning there was a smell of hot brakes coming into the passenger compartment. It was about 7 miles of town driving and I was using the brakes as little as possible. Later that day I took the right wheel off but could not find any fault.

I drove home but stopped several times to let the brakes cool down. The wheel nuts got warm but there was no smell.

I changed the left hand side disc and pads today. I drove it about a mile and the wheel nuts were again warm and the disc scalding hot.

I have changed disc and pads before on my previous XM but did not notice this problem.

What else can I do to identify the problem?

Will this go away or is something wrong?

[Jan-hendrik]

The brakes are binding.
Did you push the pistons back into the cylinders by rotating them clockwise?
Or the parking brake mechanism adjustment is out of whack.

[kenhall1202]

Assuming that the pad replacement and handbrake adjustment are both OK then another possible cause of binding front brakes could be partial seizure of the brake doseur valve operating lever which sits in the scuttle below the windscreen. Failure of the lever to fully retract when the brakes are 'off' can mean that there is still pressure on the doseur valve 'button' and the brakes will drag leading to overheated hubs.

The remedy involves removal of the scuttle cover and freeing off + lubrication of the operating lever quadrant. Also there should be a clearance at rest between the lever and the doseur valve button.

Ken

[xmexclusive]

The parking brake is designed to auto adjust.
It does this very gradually as needed during routine brake applications.

After a pad changes there is a quick sequence while the parking brake sets back up.
Having odd thickness pads one side can mess up the auto adjust.
This can cause brake binding.

The other possibility is to put the pads in the wrong way round.
The parking brake adjust works off a metal pip in the centre back of the pad.
It is essential that a pad with a pip goes against the piston side of the caliper.
If a plain pad is put piston side there is nothing to lock the pad for the auto adjust.
So the auto adjust just keeps winding up the pads on the disk with each brake application.
You end up with red hot smoking brakes.
The piston has four slots in the raised outer edge for the pips to lock into.
That lock together is the force that stops the auto adjust from tightening any more.

Genuine Citroen pads were supplied with pips on all pads so you cannot fit the wrong ones.

Most aftermarket XM ones have two plain pads and two with pips.
So it is important to sort out the pip ones and put one piston side in each caliper.
There is no guidance on this with pad sets.
There is a 1 in 4 chance of getting it right if you do not know about it.
Both pips in one caliper will bind one brake.
A pip non piston side will bind one brake.
Both pips non piston side will bind both brakes.

Avoid at all costs Pug 605 pad sets.
These are the same physical size as XM ones but come without pips on any pads.
So will bind both brakes if fitted on an XM.

To solve any of these problems I suggest you refit your pads checking the pip positions.
The pistons will need a wind back anyway to restart the parking brake auto adjust.

I have just seen Ken's post.
If there was not brake binding before the new pads were fitted leave the doseur check until you are sure the pads and calipers are correct.

John

This post has been edited by xmexclusive on January 29, 2014 12:19 pm

[BrendanM]

Many thanks for your replies.

I followed the procedure in the Haynes manual.
I wound back both pistons and fitted Mintex pads which have pips on all 4 pads.
I had to rotate the piston so that the cutouts would engage with the pips on the inner pads.

The brakes did not bind when I checked them with the engine off.

I might check them again with the engine running in neutral.

I will check the doseur valve.

What is connected to the brake pedal? A valve ?

When I wind in the brake piston what happens to the hydraulic fluid? Could it raise the fluid pressure and 'blow' a seal?

Can you tell from 'pedal feel' that there is air in the braking system?

[kenhall1202]

QUOTE (BrendanM @ January 29, 2014 02:22 pm) 

Many thanks for your replies.

I followed the procedure in the Haynes manual.
I wound back both pistons and fitted Mintex pads which have pips on all 4 pads.
I had to rotate the piston so that the cutouts would engage with the pips on the inner pads.

The brakes did not bind when I checked them with the engine off.

I might check them again with the engine running in neutral.

I will check the doseur valve.

What is connected to the brake pedal?  A valve ?

When I wind in the brake piston what happens to the hydraulic fluid?  Could it raise the fluid pressure and 'blow' a seal?

Can you tell from 'pedal feel' that there is air in the braking system?

I would make sure that the extra 'pips' on the non piston side are not fouling the caliper yoke preventing flat seating of the pad backing. Pips could easily be removed with an angle grinder.

Checking the front brakes for drag with the engine on and off is good idea and would help confirm a possible doseur / lever problem. As John says leave doseur examination until all other possibilities are exhausted. Getting to the doseur is a pain and risks broken scuttle end caps depending on how well they are attached.

The brake pedal basically operates a vertical connection bearing on the scuttle mounted quadrant lever which converts the movement to the horizontal thus pressing the doseur valve button. A light spring retracts the quadrant lever when the brake pedal is released.

Winding back the piston normally returns excess LHM to the reservoir via the doseur valve return.

Air in the braking system can cause a delay in application of the brakes (as the air is compressed by fluid) but it is unlikely to be detected by pedal feel.

Ken

This post has been edited by kenhall1202 on January 29, 2014 06:43 pm