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MG MGA - Brake in-balance

My car failed the annual MOT test yesterday because of out of balance rear brakes.

When I got the car home I checked and the nearside rear piston operates before the off side one which appears to be the cause of the problem.

Both pistons move out and return without sticking so I'm at a loss to know what the cure is.

As always I welcome your input

Regards...John
J Bray

If both pistons seem to move freely and the hydraulic pressure on both pistons must be the same, it appears that the movement of the brake shoes is the problem.
Check for some restriction of the shoe movement, and that the return springs on both sides are identical and fitted in the correct position at each end of each spring.
It would not be a faulty flexible hose as both sides use the same single hose.

Mick
Mick Anderson

Hi John

Your car cannot fail its MOT for 'out of balance rear brakes' This is not in the testers manual. The rear brakes can record different reading when applied, the only reason to fail is if the brake effort readings do not rise and fall at the same rate indicating a brake fault. What are brake readings? Obviously they can fail if partially siezed ( restricted movement ) Rear brakes can record high and low readings on the same axle, a low reading is only a failure is it is not a significant amount of brake effort for that type of vehicle, usually about 40kgs or less. If your readings are significant and all else is OK ( no leaks or binding ) then they should pass. Check out the VOSA website, I think the tester manual is on it.

Regarding your cylinders, check the handbrake lever pivot are moving freely along with the shuttle that the adjuster sits on


Terry
Terry Drinkwater

Terry

When I posted during the winter about one of my back brakes locking up you recommended swapping the drums over. It certainly improved the situation for me. Notwithstanding what you have just said (and you are an MOT tester), is this a viable option for John?

Steve
Steve Gyles

Terry

Thanks for the info

I have no problem with the MOT failure as like most examiners they are very competent.

The refusal of a certificate was for:
Rear Brake application uneven and nearside rear brake grabbing.

When I took them apart the inside of the N/S drum was very sticky almost like glue but I have no idea where it came from. When I checked the piston movment the N/S was moving first, so apart from cleaning everything and checking its free, all I have been able to do is bleed the brakes which do now appear to operate together.

One thing I have noticed is that on the N/S adjustment is complete after only 8 clicks, where as on the O/S it requires 13 clicks to gat to the same point.

Can you elaborate on the shuttle, I'm not quite sure what it is?

Steve/Terry
I will try swapping the drums over

Regards...JB
J Bray

Are the brake shoe return springs on both sides of the same strength? If they are not then one brake would operate before the other in the manner you describe though I doubt that it would matter as things would presumably even up as soon as the shoes hit the drum.
Keith Morris

I daresay the locking of a brake with sticky stuff is a leaking axle seal. Clean it up and it will probably pass the test, but the problem will return unless you fix the leak.
Tom

I agree with Tom. My first MGA, a 58 coupe, had rear brakes that were quite grabby on one side. I found that heating up the brakes got rid of the grabbiness. When I finally pulled the drum off I found the shoes coated with gear oil. This was back in 1975 and the car still had leather rear axle seals. I replaced both rear axle seals and the brake shoes that were contaminated. I also cleaned off all the oil residue.
It was from this experience that I learned that the CR in CR brand seals originally meant Chicago Rawhide.
The term shuttle above refers to the sliding action of the wheel cylinder/E-brake mechanism. There is only the one rear wheel cylinder and it must slide freely to apply both brake shoes evenly.
R J Brown

I have a frequent problem with the front (drum) brakes being out of balance on my MGA. The steps that I have found to be effective are:

Check that the wheel cylinders are operating

Make sure that the drums and linings are clean

Adjust the linings against the drum

Make sure that the lines are bled.

It has been suggested to me that backing of the adjustment on the stronger side can help. This _should_not_ make much difference as the hydraulic system will balance out the effort.
Dan Smithers

Hi John

The shuttle is probably an incorrect term, it's listed as a piston. It is the upper one that is operated by the handbrake lever, the lower piston is hydrualicly operated. The top one get all the muck and crud that builds up around the shoes. Your grabbing could be caused by the 'glue' Bostic or Super?
Hope it passes the MOt test

Terry
Terry Drinkwater

John
I would echo the others statments on the brakes. I find that cleaning them up usually gets rid of any problems.
I also had brake grab and found that I had a rear axle seal failure due to some corrosion on the sealing part of the shaft. After a solving this it worked fine.
I would suggest however taking off and cleaning the entire mechanism as the oil gum can cause the shoes to be restricted and make one grab before the other comes into play. This may also be the cause of your adjuster difference.

Neil
Neil Purves

Thank you all for your help and assistance, the car now has its MOT certificate for another year.

I'm still not 100% sure what the exact problem was, I cleaned and bled everything, made sure the shuttle was moving up and down evenly and checked the springs. If there is a leak from the axle its very small and at the moment I'm unable to find it.

I swapped the drums over which made no difference, so I then swapped the shoes over and the O/S wheel started to drag a little. Having looked very carefully at everything I'm of the opinion that the friction lining on the shoes may not be even as it looks a little thinner near the centre. The shoes are the modern bonded type rather than the riveted ones and have only done about 1,000 miles.

I'll keep any eye on it and do some shoe thickness measurments.

Thanks again for your inpot and expertise.

Regards...John
J Bray

This thread was discussed between 24/07/2007 and 25/07/2007

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