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MG MGB Technical - Brake Imbalance


Sorry to invade your board but I'm desperate. I have an mgc. I have posted on the other board but I wanted to reach a wider audience. I've searched all the archives but no joy so far.

I'm running out of ideas. I took my car for an MOT test earlier this week and it failed due to a lack of braking effort on the front right hand side. So far I have changed the flexi hose, split the caliper removed any signs of rust(not much present) replaced the pistons and seals which were new only 7000 miles ago and put it all back together again.

I think it is better now but it still isn't right. The LHS still locks up before the RHS.

The system is a single line system. The pipes for each calliper start from the same splitter. I plan to leave the car over night and see if I can get any more air out of the rebuilt caliper but I don't think that is the problem.

Any bright ideas would be welcome. Has anyone experienced anything similar


Did you do the same service to the left side?


Years ago we had a Toyota that started pulling heavily to the left under braking. I had tried everything - rebuilding calipers, replacing hoses, etc. No joy. Out of desperation I swapped the pads from side to side - Bingo it then pulled to the right! Something in the manufacturing had caused a difference in the coefficient of friction about half way through the material. They were fine originally.

The only other cause I can think of (other than air in the system) is a partially blocked hard brake line. Replacement should fix that.

Good luck,
Tom Sotomayor

Can you get good flow through the bleed nipples both sides? If not, then seeing you have changed the flex hose on the low-effort side (I presume they didn't tell you the wrong side was faulty, or you changed the wrong side) then I check for a blocked brake pipe from the splitter to the caliper.

If you get good flow both sides I'd push the pistons back into the bores both sides, then have someone slowly press the brake pedal while you watch the pistons. It's quite likely that one will start to move before the other, but you should be able to stop that moving with your hands and then further pedal movement should start the other caliper pistons moving. If they don't, and the pads of the caliper that moved first have to be pressing hard on the disc before the other caliper pistons start moving, then for whatever reason that caliper is faulty. In which case change them as a pair.
Paul Hunt 2

A bit more information

I have already swapped the pads over and the fault did not move so for my mney that has eliminated the pads

I have only interfered with the rhs calliper. The MOT station reports the LHS is operating normally and this tallies with my experience that the car steers left when the brakes are applied. It crossed my mind that what I am experiencing is similar to what happens when the shackle bolts are not tight enough on the rear axle, i.e. rear wheel steering but it doesn't explain the imbalance.

I think I've got good flow to the RHS calliper but I will have to check the LHS. I will then recruit another assistant so we can judge whether the callipers are moving in synchrony. I don't want it to be the callipers because they are only available as reconditioned units and even then they are your own units reworked for 75.00 a side.

Life was cheaper when I was a B owner



what are your discs like
try changing sides just in case thers a prob there
Ste Brown

An assistant would be a good idea for checking operation of both sides.

And maybe you do have a point of there being a misdiagnosis here, possibly a tie rod end instead? The pulling on that side may be from the wheel turning out a bit from the braking force.

Don't discount the rear brakes either. If that side is grabbing, it might pull that way somewhat. I would think it would lock up and be evident it is the rear, but who knows?

I'm thinking tie rod end, though.

The MOT machine is testing brake effort at each wheel individually, so it's almost bound to be a brake problem. (Pulling on the road could be something else, but we're presumably talking about on the rollers here).

If you simply want to get through the MoT (which is obviously not recommended, but may give you time to sort out the problem off-line) then you can ask them to test the brakes on the road rather than the rollers (you can do this if, for example, your car is a bit too low to get in the rollers). Although if it pulls hard then they're going to fail this test too.... BTW, I prefer the roller test, to the extent that I remove the front spoiler for MOT tests...

Even if there is a partial blockage in the pipe then the steady-state hydraulic pressure is going to be the same... so I wouldn't think that's an issue.

Have you taken it for an MOT (or at least on the rollers) since the caliper rebuild? One side will always lock 'slightly' before the other unless tolerances are spot on - mine does, but is okay on the MoT test. Maybe it is actually okay now?

Other option could be a warped disc? Actually, that would be my next line of investigation. Do you get any vibration under light braking at low speed (coming to rest) or heavier braking at higer speed? If so, the disc is definitely bad. A warped disc tends to 'knock back' the brakes, or you don't get full pad to disc contact on both sides at the same time.

When you say 'split' the caliper, do you mean you split the two halves? I thought that you needed special (not widely available) seals to put it back together?

Hope something in this helps!


Different calipers side to side is another thought.

Rear axle location problems usually cause rear end steering under acceleration and decelleration, not braking, as the rear braking effort is much lower than the front.

If a hydraulic pipe is partially blocked this will cause the brakes to pull the car to one side on the road, and logically (to me at any rate) that is what the MOT test is looking for as it could cause an accident. A 'stiff' caliper piston or pistons will have the same effect.

Warped disc usually causes a shimmy at the steering wheel, at lower speeds than an unbalanced wheel, and the MOT roller test is at a very low speed so is unlikely to detect that.

The point about different calipers is a good one. Did this problem (pulling to one side on the road) gradually develop? Suddenly develop? Or has it always done it in your ownership? This last could be different calipers.
Paul Hunt 2


I plan to take it back on Monday and put it on the rollers again. I think there is still a slight imbalance but it's not bad at all.

I enlisted two assistants and we tried to watch all four pistons. They all seem to be free and move at approximately the same time now.

The discs have less than 5000 miles on them and run true. It's definitely a lack of use problem. The car has done 300 miles since the last MOT at the same garage.

Regarding splitting the callipers it's not something I would do regularly but it is possible to take them apart. I did a V8 conversion about 15 years ago and at the time the recognised way of getting a V8 calliper was half a B calliper and the bridge half of a calliper from a Triumph 2000. Put the two halfs together using the bolts from the Triumph half and you've just made yourself a V8 calliper for a tenth of the price of a new one.

There is a small o ring rubber seal where the fluid passes between the two halves. I've always been careful not to disturb it and I've never had any leaks.

The only real reason to want to take them apart is to get at any rust on the outer edge of the piston cavities. If B callipers are cheap enough it's probably not worth the effort.



A blocked pipe resists flow - but once enough fluid gets past the blockage to push the calipers into contact then pressure takes over - and pressure is equal throughout the system (or it wouldn't work, and nor would your test involcing watching the calipers).

You are right about the 'shimmy', but it will also have an effect on the maximum braking force.


The only time the pressure will be equal throughout the system is when there is pressure but no further flow. When the caliper pistons are pushed back and moving the ones on the unrestricted pipe will move sooner and faster than the ones on the restricted pipe. Fluid will flow more where it is easiest to do so.
Paul Hunt 2

It's Pascal's Law. Fluid will flow to the caliper which has the 'unrestricted' pipe, but as soon as there is some resistance then the fluid prefers to go somewhere else ie the other caliper. Unless the blockage is really severe, then that's going to happen pretty quickly. The MoT test is a steady state test, not a quick jab on the brakes.

the mot test will pick up a judder on a warped disc causing a slight flicker on the gauge under light pedal
it still could be a disc that is warped maybe due to the caliper sticking on and overheating it which will cause a warpage even on a low milage car
Ste Brown

Thanks for everyone's help. I got an MOT pass on Monday morning. Both front brakes reached 300 on the dial. Not sure what the units are. The rears by contrast only managed 150.

The bonus is the tester didn't charge for the retest. Mind you I do take it there every year and they do charge maximum price.

This thread was discussed between 18/07/2007 and 24/07/2007

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