[Mk2] [Turbo] MOT brake failure - brake imbalance.

[jmachling]

I'm having trouble getting my '2 through the MOT at the moment. The only thing it's failing on is excessive brake imbalance - currently 42% on the front.

However, this is ONLY for the imbalance test at c.2mph. The max braking force is fine - it's actually higher on the side that is weaker for the imbalance test.

What I have done so far -

- stripped and cleaned the calipers and carriers
- freed up and greased the slider pins
- sanded off all rust on the pads and carrier clips
- reassembled using copper grease on all bearing surfaces

No joy. Still imbalanced >40%

The only thing left is to bleed the brakes and go over the discs/pads with some wet and dry in case they're glazed.

I think the problem is probably glazing rather than air/vapour in the system as the max braking force is fine.

Any suggestions from anyone who's had a similar problem?

[2tmike]

When you cleaned the calipers and carriers, did you also check the piston condition by ejecting them (not all the way out !!) pulling back the rubber boots and looking at the pistons themselves ? They often rust and sometimes pick up a ring of crap from constant contact with the piston seal that can cause sticking

[jmachling]

Yep, pistons are moving fine. Rubber boots intact (calipers refurbed a few years ago).

[rs007]

System not bled right maybe? Air bubble in one line?

I'm not a fan of copper grease on brake parts, I know every guy and their dog swears by it, but that doesn't mean its right. It dries out, it goes sticky, like most greased it will destroy rubber seals over time. The only grease I use for metal-rubber environments is silicone grease, which lubricates the rubber and doesn't deteriorate it.

But that's just me having a rant lol, its not going to be the cause of your issue lol!

[jmachling]

Don't worry, copper grease only used on the pads on the metal on metal surfaces. High temp rubber grease used elsewhere.

[StevieL]

Give the brakes a good bleed and it should be ok, been doing this lots in work lately with the wagons I've been working on.

[jmachling]

Got a few small bubbles out of the calliper and scuffed the disc and pads up with some fine grit wet and dry. We'll see if it works

I might just get a new set of discs/pads anyway.

[2tmike]

Did you find the cause of the problem?

[jmachling]

I reckon it was either glazing or some kind of contaminant on the disc/pads. Either way, I couldn't be bothered going back and forth to the testing station so just shelled out for some new discs and pads for the front - hey presto, instant pass.

Sometimes it's just not worth the hassle stripping things down and fettling when you can replace parts with known good ones for not much money.

[2tmike]

quite agree and glad you've had success....
I've done pretty much the same but mine are still not really right...((