Odd brake pedal feel with long travel after rear disc/pad change.

[Droverunner]

This is weird... I've changed disc/pads sets probably 100+ times never having problems... and in the past given advice on forums where just the same question has been asked. But this time can't seem work out what's happening.

So... 2012 Viano with genuine 58k recorded bought a few months back. Been out of regular use for 6mths prior to purchase. Discs had the usual rust from standing... on the front it was minimal as the discs and pads were obviously very recent in mileage terms so they polished in a few trips. Rear brakes looked original with some scoring and deeper rust... they haven't polished to my satisfaction even after a few months so decided to replace. The actual braking was powerful but just to get the job ticked off I bought new Febi discs and pads.

Changed last week with no problems. Sliding caliper free but pins re-greased anyway. Everything fitted back properly. It is my habit to open the bleed nipple as I push the pistons back... I apply light positive pressure before opening the nipple and close the nipple while easing off pressure so there is no air ingress.

So job all looking good and with the engine off and pedal pumped a few times to bring the pads to the discs it came back hard. However on taking it for a run I immediately... as in getting out of our road... noticed far more pedal travel than before. I've had this with new pads on old discs before some miles bedding in but not with all new items. I ran it several 10mls journeys but no improvement.

As far as I was concerned there was no way I'd got air in the system somewhat proved by a hard unassisted pedal but I wanted to change the fluid anyway so did that this weekend. As expected the old fluid was clean and free of any air. No change in the long travel when servo assisted for bleeding. After bleeding I got Mrs D to sit with foot on the pedal engine running... no leaks, bulging hoses or any other visible issues.

So the pedal feel travel on the drive is... Unassisted a couple of mm mechanical play and then no more than 1cm travel with firm pressure and stays there. Assisted same mechanical play but 7cm of travel as if it's falling away from your foot with only modest resistance rather than the spongy air in system feeling. If driven normally with no regard to the pedal feel it does stop OK but if at traffic lights this pedal travel when its held on is not how it was and feel wrong.

My iCarsoft i980 doesn't produce braking system codes.

Any thoughts appreciated.

 

[curious]

Did you pedal pump several small short strokes or fewer long strokes?

 

[Droverunner]

If you mean bringing the pads to the discs both being new there was little slack to take up so a couple of short pumps did it. If you meant while bleeding I used the pressure bleed kit so didn‘t pump the pedal. Don’t be diverted by the bleeding aspect though, I didn’t introduce air doing the pads and the routinely needed full bleed made it neither better nor worse.

 

[curious]

I had enormous problems with the symptoms you describe (thread somewhere on the forum) and changed the master cyclinder (twice!) thinking I'd potentially damaged the seal when taking up the slack by using too long a stroke. It wasn't that.

In the end it was corrosion on the pad carriers that was causing slight impedement of the pad movement. It doesn't look like that for you given the condition of them in your picture.

Just thinking further, are they shot blasted or have you painted them? Their condition looks remarkably good. If painted, the paint my have reduced the gap where the stainless shims go. (It was specifically the expanded rust either side of the shims in mine that was the problem.) Struggling to believe paint would cause a problem but you never know.

 

[M80]

Sorry no technical suggestions.
The rears can be a bitch to bleed, but you shouldn't need to worry about that

Work them with aggression for a day or two,
then consider, if required, pedal bleeding the rears again.

I've tried traditional ways and then assumed the master cylinder was goosed, but they came right.

I know it sounds very amateur.

 

[Droverunner]

All good thoughts thanks and yes curious your thread was one of the two I found on here.

I did paint the calipers which were rust free but discoloured however with the lightest spray and before putting the stainless "springs" in I wire brushed the paint back off that channel the shims sit in. I did check the pads moved freely in the shims.

I'd started to think of long shot ideas like something had happened with the brake assist system or the valving that controls rear brake pressure for anti rear wheel lockup.

M80 those are the lines I was thinking along as neither will be any trouble or cost anything but then my next move I think will be to take it to the village MOT station and pay for an MOT style brake test and get their opinion on the readings and pedal feel.

 

[AnthonyUK]

If the pad fit is fine then my guess is it will be air.
Simplest answer for what is in effect a simple hydraulic system.

Try another bleed before spending money.

 

[Droverunner]

>>>Try another bleed before spending money.

Yes will do... I'm 99% sure it isn't air but for that remaining 1% has to be done.

My thoughts are leaning towards the brake assist system but I have no details on it.

 

[Botus]

where's the park brake - shoes inside the disc hub - or wind back the brake piston to get some space ?
if its on the caliper they should self adjust and take up the slop -

 

[Droverunner]

Shoes inside the disc hub. They were almost as new and with the new discs and proper set up that bit works very well.

Since my last post I've done a lot of cross country miles and not that they needed bedding in that element can be crossed off... no difference.

 

[M80]

I'm sure you will have checked, but I'll ask anyway just in case.
Are the calipers and pads sliding freely?

 

[Droverunner]

Yes beautifully so. Rust free calipers pre paint. Paint removed from shim contact points. Shims polished and pad holding contact points Copaslip coated.

Brakes hard tested several times and will bring in ABS on a dry road. It's just the weird pedal travel at rest.

 

[M80]

Another dumb Q,
coper slip no where near the rubbers?
It causes rubber to swell.

 

[Frontstep]

My thoughts are with the master cylinder.

 

[Droverunner]

No Copaslip on the rubbers... all very neatly done.

Re the master cylinder thoughts have to turn to that area of master cylinder/servo/ABS. As far as I know it has brake assist whereby under certain circumstances the electronics take over and decide braking forces.... possibly using the ABS pump. I wonder if STAR diagnostics would pick up up anything on this system?

Next step is this weekend to bleed again. After that to the village MOT garage where I know them... I'll get it roller tested and see how they think it feels.

 

[Botus]

sometimes everything working means there a bit of space to take up before the pads bite - when it was old seized and didn't move there may heva been less slack to take up

 

[joderest]

guessing here.
When you pushed the pistons back, could it be you may have damaged the internal seals in the Callipers ?
That may have caused some air to be trapped in the Calliper.

 

[Droverunner]

Well the vehicle's had enough use to bed in new brakes which in truth hardly need bedding in when both discs and pads are new. I've also been through the bleeding process again... not even a tiny bubble. Stripped the rear brakes again and everything perfect... easy pad movement in holders... smooth easy caliper piston movement... no hose bulging... and no excess movement seen around calipers when brakes applied hard at rest.

Road testing will easily bring it to lock up and ABS operation in the dry... at all speeds and all degrees of braking strength it feels totally secure in braking. Just this unsettling travel at rest with engine on. Last check is the roller test at the MOT station and I've not had a chance to drop in for that but will do soon.

As an aside something I've never had to resolve in quite the same way before...

Noticed an annoying "groan" from one rear wheel on pulling away from a fuel pump... and version of same noise at 50mph like a musical tone. Not like the squeak of a sticking pad or one that needs damping compound on the back.

Looked at this when re-checking the brakes and nothing seen to cause it. But some sixth sense made me flick the discs dirt shield and it rang with a similar tone. Fitted tightly and in good condition. So I put some blobs of silicone on the rear of the shield until flicking it gave a dead response. Now it's all quiet on the road.

 

[M80]

Sorry I'm travelling across 2 threads here for the same issue.

Another thought, how is your vacuum pump?

 

[Droverunner]

Re the bleeding. I *know* I didn’t introduce air when changing discs and pads… as proved when I bled mainly to routinely change the brake fluid and no air seen. But I re-bled just to be sure. Add to that a rock hard pedal once assistance is exhausted with engine off and I think air is eliminated.

Re vac pump I assume not faulty as assistance is powerful and even when driving.