MOT Brake imbalance

[NOEL]

Need some info from some of you experts,took my minor for its MOT, it failed on one item brake imbalance, I changed one of the brake cylinders on the front near side as the tester said this is where the problem was, wish I had done both now but I could see one had a perished seal and assumed the other was ok,anyway, took it back and although its improved its still not satisfactory, everything else has passed so this is a blow, the brake analysis showed
brake force - kgf
left right axle total
1 sb 244 156 400 max imbalance 46%
2 sb 168 191 359
the manual evaluation has bind/judder and grab rate of incease and decrease as passed in all areas. I am going to change the other cylinder and re bleed it but can anyone explain how the percentage is calculated and do you think the cylinder change will do the trick? the shoes are fairly new-couple of hundered miles at the most, no visible leaks and a good pedal.
Thanks in advance for any help

[alainmoran]

If you had a leaky seal then the shoes will be suspect ... brake fluid will have seeped into the lining and stuffed it - that's probably where the judder is coming from ;)

[NOEL]

Sorry, I did not make it clear it has not got judder, judder and grab have both passed on left and right,the shoes were bone dry when I took off the drum and no sign of fluid weeping anywhere

[beero]

New shoes both sides and brake cleaner for the drums would be the best place to start. Then renew the other wheel cylinder and also the flexi rubber pipes, they can collapse internally and cause a restiction and imbalance. Don't skimp on brakes.

[bmcecosse]

Just check if the cylinders are seized.......by turning the piston in the bore. Can you feel the car pull to one side when you brake hard? If the pistons are all free then it's also worth swapping the shoes and/or drums side to side to see if the 'pull' follows one or the other. And I have in the past had to split the set - so one left shoe swapped with a right shoe to get a good balance. Hope your tester is 'ok' and gives you another free re-test!

[autolycus]

[quote="NOEL"]
the brake analysis showed
brake force - kgf
left right axle total
1 sb 244 156 400 max imbalance 46%
2 sb 168 191 359
/quote]

1 = front axle, 2 = rear axle
sb = service brake, i.e. footbrake

The numbers are the maximum brake force (haven't these VOSA peasants heard of Newtons as units of force?). The maximum achieved on the front offside is thus well below that achieved on the nearside, so I'd suspect a problem with the front offside - probably a completely seized cylinder.

The imbalance figure is measured continuously as the tester increases the pressure on the brake pedal, and is calculated, also continuously, as the difference between the higher and lower force divided by the higher force, expressed as a percentage. 25% is the maximum imbalance allowed at any time while the pressure is being increased, not just at the final maximum point.

I don't know the weight of your particular car, but you'd actually have achieved enough total brake force if the front left maximum had been as low as the front right maximum - if you can get both fronts up to 244kgf, you'd have excellent brakes.

Kevin

[bmcecosse]

Great explanation there ! Thanks -I didn't know that -although I could see the two readings added up to the total for each axle. And yes - serious problem on the front driver side, and some problem on the rear nearside, although I believe the inbalance there is not measured? R

[mike.perry]

Remove the brake drum, press the brake pedal and visually check that both pistons are moving.

[NOEL]

Thanks for the advice, I am going to replace the old cylinder on the front nearside to match the existing new cylinder on the same drum. I will try new shoes also as it is possible that they could have been contaminated even though everything is dry. the car pulls in a straight line so If pulled to one side I would have noticed it earlier, one point,the discription right and left I assumed was as viewed from the front of the car, this would match the tester saying it was the nearside front brake that was the issue. IF its the other side then I would need to change the cylinders on the drivers side instead

[autolycus]

Right and left are as viewed in direction of travel, i.e. from driver's seat looking forward. So RH = off side = driver's side, LH = near side = passenger side.

But why are you reluctant to follow either Roy's or Mike's advice about checking whether cylinders are seized before you think about replacing them? When those two agree on something, there's a very good bet they're spot on! Colour as well as dryness is a good clue as to whether linings have been contaminated. If they are very glazed, roughen up the surface with a bit of coarse emery paper. If they're very old linings (20+ years), be aware that they may contain asbestos.

Rear brake imbalance doesn't have a numerical limit. The Testers Manual Reason for Rejection is:

c. the braking efforts at the road wheels do not
increase at about the same rate when the
service brake is applied gradually.

Kevin

[mike.perry]

Cheers for the vote of confidence Kevin
I am a firm believer in checking one thing at a time, it saves a lot of money

[bmcecosse]

Seriously - it's the DRIVER side front brake that is not working well - one of the cylinders is probably seized. Easy to check them!

[LouiseM]

My Minor also failed it's MOT this week due to a brake imbalance. A new cylinder & rubber hose sorted the problem.

[mike.perry]

I bet that you checked to see which cylinder had siezed before stripping everything apart.

[NOEL]

Thanks for all your help, its not that I am reluctant to check the cylinders, its more based on the fact the MOT man told me that it was one particular side that had the problem, anyway, will follow the advice of checking all cylinders but will obtain some more spares and probably end up changing all of them,

[bmcecosse]

Put it this way - the cylinder you have already changed - was it seized??

[mike.perry]

Some people like making work for themselves