Front Brake Imbalance

[GLPoomobile 958]

1.6 GTI still with drums at the rear and obviously the 1.6 compensator.

 

Brakes were lovely before - OE 1.6 front calipers (Bendix?) with Black Diamond discs and Mintex pads.

 

What I've done:

Brand new Bendix MC, 20mm (yes 20mm, not 20.6)

New OE reservoir

Lucas (Girling) 306 XSi calipers from Anthony

New ATE Powerdics - 266mm

New Roulands (pattern) pads (from Brakeparts)

 

First problem was a long pedal. After mucho bleeding the brakes worked, and I'm used to the pedal now.

 

Took it for the MOT, and it only just scraped through the brake test. Front brakes imbalanced. With all the new parts, what could it be?

 

Remembered that NS caliper piston was tough to push back to fit the pads. The OS one I squeezed back by hand, but the NS I had to lay on it's back and lay the handle of my hammer over the piston and lean on it.

 

Anthony said this:

As for the front brakes, I don't think that they should need refurbing - they were definately fine (or atleast they pulled up straight) when they were removed from the car, and being sat around in the dry shouldn't really have done them any harm. Worth just double checking the sliders are still free, exercising the piston a bit (push it in, then press the brake pedal to move it out, and repeat a couple of times) and letting the disks and pads bed in fully before going down the refurb route

 

I decided to buy 2 piston repair kits anyway, and rebuild both calipers. As it happened, I ended up only doing the NS though. Never done it before, but the piston, cylinder bore, and sliders all looked pretty good. Gave them a clean with 600/800/1000 grit wet 'n' dry. Piston slid fine in the cylinder, but was very tight with the seal fitted, just like it was before.

 

Fitted back together. Bled the system a few more times. Went for test run. The roads were damp, and under medium/heavy braking the OS front is locking. I don't slam on the brakes, I do it nice and progressively, so the locking doesn't happen immediately. Luckily with the long travel I can modulate it easily and release as soon as I feel it lock, but it needs sorted.

 

Have done various searches, but most topics are about rear wheels locking. The only 2 things not renewed are the servo (not gonna be that) and the hoses (were fine before). Hoses are easy and cheap enough to replace, so happy to do that.

 

But I wonder, could it be the MC? Is it likely that a brand new Bendix MC could be at fault? One thing I noticed is some fluid damage to the servo since I fitted the MC. I had cleaned and painted the servo, and the paint has bubbled right around the edge where the MC mounts, and there s a trail below. But there is no sign of continued leaking, and I know I spilled a little bit of fluid, so not sure if it's just where it's tracked down the MC body and onto the servo after that. The seals are brand new and no sign at all of any leaking.

 

Or, could I be getting my logic arse about face? Could it be the OS caliper that is to blame? The NS piston was stiff to retract before, and is the same with a new seal, so that must be how it's supposed to be. If the OS caliper piston is moving more freely, perhaps it is more easily able to apply it's pressure to the disc and is doing it's job earlier than the NS caliper. Guess I'd have to test by hitting the brakes quick rather than easing them on, and see if both sides lock.

 

Sorry for the essay (as usual). Any ideas?

Edited October 15, 2008 by GLPoomobile

[saveloy 8]

Mate, I'm about to solve both your problems in one hit. And it isn't good news.

The reason you are locking the OS front is because you have not meddled with it. Ironically you have created the problem. By cleaning up the NS caliper you have created less friction between the piston and caliper bearing surface. It may be tight on the seal, but my bet is that you have created a bigger tolerance between the piston and the aliper wall. Although this means that the piston will push out easier, it also means that it will suck back in easier too. So, you now have an inbalance between the calipers. The OS will be on the pad first, applying pressure and causing the OS wheel to lock up first. The NS piston is, in effect, playing catch up.

This is also the likely cause of your long pedal - too much piston retraction. Add a smaller MC bore and you can understand why you are in your present predicament.

Unfortunately, the only solution is a new caliper. Unless you fancy ruining the OS caliper for a bit of a balance.

The leaky MC reservoir, on the other hand, is very common. It isn't a big issue. Fit new rubber seals, if you haven't done so already and put a cable tie around it to force it towards the MC.

Edited October 16, 2008 by saveloy

[GLPoomobile 958]

You obviously skim read my post. My fault, I type too much!

 

The imbalance was there BEFORE I took the caliper apart. When westy was helping me bleed the brakes, he pointed out that the car was diving on OS front when I drove forward and hit the brakes.

 

And as far as my 'honing' attempt of the parts goes, I'm a lazy sod, so spent the bare minimum time sanding them. I doubt I've taken that many microns off each surface.

 

And the reservoir is fitted to the MC with new seals, and held down by a cable tie.

 

Next!

[GLPoomobile 958]

Also your theory is not sound.

 

You can only guess at what the tolerances are between piston and cylinder on each caliper. I said that the OS piston moves more freely (even after rebuiling the NS one) so surely that one would be both moving out AND retracting more quickly than the NS one. So if the problem is at all related to the 2 theories here, it's more likely the fact that the OS is more eager to apply as the piston is more free.

Edited October 16, 2008 by GLPoomobile

[Rob Thomson 6]

I don't buy Saveloy's reason - there's no 'catch up' as the brakes don't do anything until the fluid's stopped moving. And there's bugger all movement anyway, regardless of differences between seals etc. I also don't think it's got anything to do with the MC.

 

Firstly, are you sure both calipers are the same, especially with regards to piston sizes? These car manufacturers can be sneaky bastards and use calipers that look identical but have different piston sizes on different models... Probably unlikely but worth checking.

 

There are things other than the brakes that can make one wheel lock before the other. Are you sure there's nothing wrong with the suspension? A completely f***ed damper might be enough to cause premature locking.

 

Are the rear brakes similar side to side? Pug rear suspension squats when the rear wheels are braked. If one squats more than the other because its braked more than the other, you effectively take some load off the opposite front wheel which will lock before its mate.

 

One last thing. I recently put new calipers on my 1.6. When I bolted one of 'em up to the hub-carrier I noticed I couldn't move it on its sliders. Weird as the sliders felt perfect when the caliper was off the car. I realised that the bolts weren't parallel because the hub-carrier was bent, and that was locking the caliper solid to its sliders. The old caliper wasn't affected because there was enough play in the old knackered sliders. You might have the same problem.

[saveloy 8]

I mention the 'sucking', for want of a better word, because I had this problem with a set of old 4 pot calipers. The pedal had a dead spot ot the top where the pistons were in free air. Plus, they tended to get knocked back quite easily, exacerbating the problem.

 

Why don't you pull off the wheels, push the pistons in completely, then give the brake pedal one complete shove? Go measure the protraction on both sides to see if there is any discrepancy.

[Rob Thomson 6]

Sorry, I don't mean to sound like a one-man anti-Saveloy mission, but that won't prove anything. One piston will move more than the other whether there's a 0.00001% imbalance or a 700000% imbalance between sides.

[hengti 2]

one's more likely to lock than the other in most circumstances as (apart from differences in caliper efficiency), you've got different pieces of road under either tyre and different loads, etc

 

does it actually pull the car to whichever side under heavy braking (before locking?). it might only be a problem if it does; and, even then, depends upon severity

[HERMAN 33]

Stupid question have you done any thing with you wheels? odd tyres may cause this. Try swopping the wheels over and check pressures. Also are you happy that it is properly bleed. A tiny amount of air in the system could cause alsorts of problems (have you bleed the backs also?). I remember also having a big problem with a car at mot with a front brake imbalance. This was caused by me cleaning the anti corosion grease off the new disc on my side propperly and my mate had only done a half hearted job when we had changed the disc's and pads. This had poluted the pads with grease on one side. We cured the problem by driving with a foot on the brakes to get the new pads nice and hot to burn off the crap. I have also had it before were a set of new pads had had two differant makes of pad in the box. Which could of caused a problem but I was lucky to have noticed.

Hope to have been of some help

Neil

[saveloy 8]

Sorry, I don't mean to sound like a one-man anti-Saveloy mission, but that won't prove anything. One piston will move more than the other whether there's a 0.00001% imbalance or a 700000% imbalance between sides.

 

Rob, that's fine. But why won't it prove anything? I'm simply trying to evaluate whether there is an obvious problem with one side (notably, the NS).

If there is a real problem, a fair disparity in the protrusion will prove a problematic caliper.

If we progress logically, then surely that dictates that we find the problem concerning the NS.

[Rob Thomson 6]

What I'm getting at is that the forces required to move the pistons in their bores is tiny compared to the forces needed to produce any significant braking, let alone for that braking to cause a wheel to lock. I bet if you went out and bought two brand new calipers and did your test you'd find that one piston moved significantly before - and more than - the other.

Edited October 17, 2008 by Rob Thomson

[saveloy 8]

Rob, I understand that.

But we are talking about a set up with a distinct problem. I've had many an occasion, with a previous set of calipers, where a good shove of the brake pedal has made a movement at one side, and nothing at the other. It was no surprise to find a partially seized piston on one caliper.

[welshpug 1,657]

so you've rebuilt one but not the other?

 

try rebuilding the other side...

[GLPoomobile 958]

so you've rebuilt one but not the other?

 

try rebuilding the other side...

 

The original plan was to do both, for the sake of it, hence I bought 2 seal kits. When it came to it, I wisely chose to do one at a time, starting with the NS, which I suspected to be the cause of the fault. I only managed to do that one side, due to struggling with getting the piston out and then getting the dust seal back on during rebuild.

 

So, I see your logic in suggesting I rebuild the OS, BUT, if the OS piston was moving more freely than that of the NS caliper, what benefit would there be in rebuilding it? It's obviously not seized/sticking.

 

Anyway, I did some brake testing while out yesterday (took a drive down the A7 from Edinburgh to Galashiels, along the A72 to Peebles, and back up the A703, for anyone who may be interested), and the results where

 

1 - It doesn't pull to one side

2 - Braking from 70mph on a reasonably dry (ish), smooth surface, it pulls up really nicely, and only locks a wheel* as it's almost coming to a halt.

3 - *Repeated tests had it locking the NS wheel a few times, and both wheels together on another time. Could be due to the surface I was on etc

 

Basically I'm actually reasonably happy with how it's stopping, and were it not for the result of the MOT I probably wouldn't have even thought this was an issue! I might give it some more miles, let the brakes settle in more (as Anthony suggested) and then get the brakes tested again to see if there is still an imbalance showing.

[saveloy 8]

Conclusion: there actually isn't one!

I would suggest you apply some Lucas rubber grease beneath the outer seal of the NS caliper you played with. I know you are going to be very reluctant to even go anywhere near it, but it is a wise move, since you've scrubbed away under there.