01 brake imbalance MOT failure, no diagnosis - gnucar
My '51 106 (main dealer FSH) has now failed its MOT twice on brake imbalance, despite new pads between failures (didn't need them, they were pitted but thought it might cure it, its only done 30k so on original pads)- calipers look fine, move fine, pistons move with thumb pressure, discs look ok, hoses look fine, no judder on braking. Its giving a 36% imbalance (passenger side better) on first pressure which drops to 12% ish, but the tester wouldn't pass it. Chaged the discs over and it dropped to 25% Any ideas? Everything else is perfect, in every way, just this issue. It does seem to be pulling to the left on braking, more so on harder braking.

{Moved across to Technical Matters}

Edited by Dynamic Dave on 13/10/2009 at 14:07

pug 106 brake imbalance MOT failure, no diagnosis - b308
Bleed the whole brake system?

If that doesn't work, then I'd be looking at the callipers next... even though they look ok it doesn't mean they are working properly... have you tried a decent local Independant to have a professional look at them?
pug 106 brake imbalance MOT failure, no diagnosis - OldSock
There may be a small amount of air in the hydraulic system. Has the fluid been changed recently and the system not bled correctly?

Discs and pads do need a bit of time to 'bed in', which sometimes gives unbalanced readings - particularly under light braking.
pug 106 brake imbalance MOT failure, no diagnosis - gnucar
Thanks, I'm thinking calipers. Sigh, I though it'd walk it so put in in the council place. Its at my decent indy guy now ....
pug 106 brake imbalance MOT failure, no diagnosis - Number_Cruncher
I think it's very unlikely to be air in the system - consider the physics of how fluids exert pressure.

What I would do, instead of guessing, is to make up 2 temporary brake pipes, from the inboard end of the flexi to the next component back in the chain (ABS module?), but, cross the pipes, bleeding through as normal.

If the problem changes side, you KNOW the problem is upstream of your crossed pipes. (ABS and master cylinder)

If the problem stays on the current side, you KNOW the problem is confined within that wheelarch.

Obviously, don't use the car on the road with the non standard brake pipes - it's purely for diagnosis.

pug 106 brake imbalance MOT failure, no diagnosis - Falkirk Bairn
Similar to my DiL Mini a few years back.

3 years old, just had service booked stamped as serviced under £500/5 year TLC @ a BMW garage. 1 month later son said no point travelling 15 miles, go to local indie for MoT

Uneven braking failed the MoT despite being under 3 years and "just serviced" or rather just had service book stamped. New caliper required on one side - normal braking was OK only under stress of hard braking did the fault show up.

BMW garage excuse - just an oil service and that was a month ago - so in 3 years it had not had the wheels off/brakes cleaned out/piston examined yet it had a full BMW Mini service record - a joke - car sold and new Honda bought.
pug 106 brake imbalance MOT failure, no diagnosis - Altea Ego
BMW garage excuse - just an oil service and that was a month ago -
so in 3 years it had not had the wheels off/brakes cleaned out/piston examined yet
it had a full BMW Mini service record - a joke - car sold and
new Honda bought.


It wasnt in your service sceduled as published in the service book, and you would have complained about the extra 100 quid labour to do it had they done so.
pug 106 brake imbalance MOT failure, no diagnosis - Bill Payer
new Honda bought.

That will be exactly the same.

The only car I know where they routinely service the brakes is Mitsubishi - and that's only the rear drum brakes, which they strip, clean and adjust the handbrake, every 2 years. Mind you, that's also the only brand I know where the brake fluid change is also included as part of the service.

On our Honda Jazz, the drums had never been off in 6 years.
pug 106 brake imbalance MOT failure, no diagnosis - pmh3
NC I like your logical approach, but just one thought, since the system will be 'split', with a separate M cylinder seal/piston for each side is it possible that air in one side of the system (or even a faulty mc seal?) will result in different effects at the calipers?
pug 106 brake imbalance MOT failure, no diagnosis - Number_Cruncher
>>will result in different effects at the calipers?

If the system is front / rear split, then, the whole of the front brakes will share the same pressure.

If the system is diagonal split (much more likely), the floating secondary piston in the master cylinder will tend to equalise pressures. If there's enough air in the system to allow the primary and secondary pistons to touch (as happens if one circuit fails), the pedal will feel absolutely awful, and the OP hasn't complained about a spongy or soft pedal.

pug 106 brake imbalance MOT failure, no diagnosis - gnucar
Thats right, the pedal isnt spongy. No ABS either

Edited by einoxon on 13/10/2009 at 14:25

pug 106 brake imbalance MOT failure, no diagnosis - 659FBE
The most likely cause of this problem is stiction in the front calipers. Pattern pads frequently jam in the "dovetail sliders" on these Bendix calipers - the inner pad has to be free to move so that Newton's Third Law will work.

I would suggest the following: Check that all pads are in reasonable shape and have the lining surface parallel to the backplate. This is important - wedge shaped pads will cause the slider pins to jam. Then check the slider pins and boots themselves for absolute freedom of movement. Finally, fit the pads and check that the inner pad is free in the "dovetail" on the carrier - this is essential. High temperature copper based anti-seize compound should be appled sparingly at these two points on the inner pad ends. Finally fit the wedges and check that the inner pad is still reasonably free.

Everything has to be right with these Bendix calipers for them to work properly - I thoroughly inspect, clean and lubricate annually.

Finally, if the vehicle has Bendix drum brakes on the back and the wheel cylinders are more than 3 years old, the trailing pistons will be seized - replace. The give-away for this condition is that you can reduce brake pedal free travel by applying the handbrake first. The auto shoe adjusters will not work with a seized wheel cylinder piston. The MOT brake test will find this fault.

659.
pug 106 brake imbalance MOT failure, no diagnosis - kithmo
Might be worth renewing the flexi-hoses as they can get damaged internally by slipshod clamping methods.
pug 106 brake imbalance MOT failure, no diagnosis - Number_Cruncher
659,

I would have thought that stiction of the caliper, and the inner pad would be causing an MOT fail for binding brakes as well as imbalance - the OP hasn't mentioned binding?

 

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