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MG MGA - I'm stumped

Right rear brake produces double the braking force of the left one

Front disc brakes produce equal force

I've wound both sides up on the cam adjuster until they lock, and backed off both two clicks. Backing off the better side another click or two does nothing.
I've bled both sides - no change
I've swapped the drums from one side to the other -no change
I've switched the shoes from one side to the other without moving the problem from one side to the other
I've checked that all is correctly assembled
The cylinders both move freely in their slots
The handbrake is also better on the better side for the footbrake
Shoes are from the same batch and box
Drums are the same age (about twp years old) and have the same casting number
Nothing is leaking or contaminated with oil
Handbrake cable is greased and moves freely
Handbrake levers also move freely in the cylinders

What am I missing ?
dominic clancy

Dominic,
Could it be the wheel cylinders? You don't mention having tried different units there. I'll be very curious to hear what the cause turns out to be.
Gerry
G T Foster

Dominic

What is the basis of saying double the braking force. Did you measure it on a rolling road?

Is it a case of more weight on the left side (driver) gives the impression that the right side is locking easier?

Tyre pressures/wear - more surface contact on one side?

Steve
Steve Gyles

Dominic,
You state that the clinder moves freely in the backplate but is the steel piston seized in alloy cylinder? That situation would prevent the shoes from moving properly - if at all.
Tony
Tony Mitchell

I measured it this morning on a brake testing machine after each change.

I don't think it can be a stuck cylinder because the brake wouldn't work at all. Working the handbrake shows everything is moving fine, and the brakes do work, just with a large imbalance
dominic clancy

Dominic, the only thing you haven't moved are the cylinders! (and you probably can't switch sides on them). Try a new cylinder in the duff side?
Art Pearse

Are you certain the shoes are installed correctly?
And got pictures (both sides as assembled)to prove it?!

FRM
FR Millmore

Hi FRM

What do you mean bvy with the shoes are installed correctly.
The springs go on the inside, what do we have to look for else?

serge
serge

Picture Left side inner


dominic clancy

Left handbrake link


dominic clancy

Middle cable linkage


dominic clancy

Right cable link


dominic clancy

Dominic

I think your brake shoes are on the wrong way round.

Steve
Steve Gyles

Right inside


dominic clancy

Side by side


dominic clancy

Steve

If that were the case, why is one side absolutely fine, the other producing only 50% of the stopping power?

This is what is stumping me.....
dominic clancy

Dominic

Now I have seen your right hand side is the same, I would agree with you.

I may have it all wrong, but it seems to me that you have the trailing edge of each shoe set up as the leading edge.

steve
Steve Gyles

Dominic

I am wrong. just read Barney's bits and looked at manual. You are set up correctly. My logical brain is incorrect on this one.

Steve
Steve Gyles

A very long shot, but could it be a suspension issue. A worn shocker could just be causing the wheel to hop slightly as it tries to stop so lessening contact with the road.

Malcolm
Malcolm Asquith

Hi there
An other very long shots,
1) my tow truck driver put his tiedown over the brake line runing next to rear axle and squash it.
2) I would disconnect the brake line at the axle tee and wheel cyl, and blow it out see what will come out
3) Why and how did you discover there is problem? It may give you starting point where and for what to look for

Martin
sharpeys

I have checked for physical damage, and can find none. When bleeding there is no shortage of fluid coming through the nipple on the poor side.

It's all apart on stands to figure out what to do.

I have known that there was a braking imbalance for a while, and with the safety test on Tuesday wanted to adjust the imbalance, which is why I went to the DIY workshop and put it on the rollers to test and fix. It manifests itself as the right brake locking up under hard braking.

It was there at the last safety test too, but I evened it out by swapping the shoes from one side to the other. Now I need to find the cause and fix it properly as that fix didn't work.


dominic clancy

Dominic

You have not mentioned the tyres. Are they equal pressures, matched pair with equal wear?

I have just been through the archives. I had an identical problem that was cured when I swapped the drums; but you have done that one.

Of course, you also have a heavier weight on the left (driver) which could affect your tests. may be put ballast on the right side and see if it makes a difference.

Steve
Steve Gyles

your pictures show the springs on the outside when they should be on the inside to pull the shoes to the backing plate..check the photo on the mg guru website
don wallenius

Could you have a step in the cylinder bore which is causing the piston to stick before full travel.

For a step to build up is not unlikely in a car which is not used for a while. Corrosion builds up on the exposed part of the cylinder and is pushed out as the brakes are applied for the first time. if this happens a few times it can cause a distinct lip of rock hard crud in the end of the cylinder.

You may be able to test for this by readjusting the brake but only back off one click. It will make the brake apply with less movement of the piston.

Paddy


P Reardon

Don, The setup is the same both sides, so even if the spring position made a difference, it should make the same difference both sides. Although I could change them to the back side of the shoes, this can't be the root cause. In any case, the beehive springs hold the shoes to the backplate, nit the return springs.

Paddy, the car is used a lot, doing about 5k miles a year. I also have used silicon fluid in the system from the start, so corrosion and wear problems are not an issue.
dominic clancy

Dominic, I was thinking on similar lines to Paddy regarding either a cylinder bore damage/corrosion issue or maybe even the bore diameters being of different sizes.
I would have thought that unequal cylinder bore sizes would probably deliver different pressures to the shoes and therefore give different braking effort.

When you think about it, you have pretty much eliminated all the other possibilities as the cylinders are the only parts you have not swapped over as far as I can see!

The only time I have ever experienced unequal braking from the rear drums on my car was when I backed off one adjuster to stop one of the shoes from rubbing very slightly.
To correct this I adjusted up the shoes til the drum locked and then backed the adjuster off just enough to allow it to rotate. I ignored the number of clicks it took, I just backed the adjuster off until the drum was free enough to rotate. The shoes still rubbed very slightly but this seemed to bed in very quickly and the braking balance at the rear was restored.

Best of luck

Colyn
Colyn Firth

I agree that everything is correctly assembled, except the springs should be between the shoes and backplate.
But, all is symmetrical, so it >should< work.

As some have pointed out, you haven't said that you swapped cylinders. While you are doing that, check that they are in fact the same diameter. There were lots of cars using that brake design and it is conceivable there are different diameters. I've also heard of rebuilders who bore worn cylinders to odd oversizes and use OS seals. Also possible that there are aftermarket cylinders with odd sized bores.

Remember that any hydraulic differences are magnified by the self-servo effects of the brake geometry, so don't expect a 50% piston area error for a 50% braking effect difference.

Be certain that the area of the backplate where the cylinder slides is clean, smooth, and lubricated. When the brakes are applied, there is an off center load applied before the cylinder centralizes under pressure, and if it sticks there it would reduce force on the shoe that is actuated by the cylinder body.

I've never had even the idea of having a brake dyno, sounds wonderful! I assume there are rollers for both sides. Is it possible to position things (turn the car around) such that you are measuring the brakes on the other rollers, as a check of accuracy?

That's the brake part, now to suspension/corner weights.

Something like a weak LHF spring and/or a twisted ARB could be reducing the weight on the RHR.

Is it possible that you are putting a "set" in the suspension before you drive onto the rollers? Say coming in at an angle over a bump like a driveway entrance, coupled with friction in the springs.

Lacking corner scales, you could try jacking up the LHF a bit to see if that affects the RHR brake efficiency.

As a final desperate bodge, you could file a long chamfer on the leading ends of the LHR shoes, to reduce the servo effect, but it's not the right way.

Best of luck, this is interesting!

FRM
FR Millmore

Hi Fletcher

Thanks for the suggestions

Brake dynos are a fact of life here, and are calibrated regularly. I did a cross check with the fronts which showed up as balanced, and as the rollers straighten out the car automatically if you are sitting at a slight angle, it's not down to an angular effect. I also only weigh 150 lbs, so the weight is not a big issue. The suspension was all checked out and all rubbers renewed when I fit the new drums and back axle two years ago, so there's no issue there. I know that the cylinders are identical because I did the seals at the same time. They were new Lockheed units when I fitted them 22 years ago, and are in great shape. I only replaced the seals as part of a regular service routine - I don't trust old rubber parts, particularly in brakes. I don't have a ARB, and the front springs also appear to be in good condition (the car sits a 1/4" higher on the right, which supports that).

I'll pull the shoes on the left side and copper-grease the cylinder in the slot tomorrow, and back off the adjuster on the other side a few clicks more to reduce its efficiency in the hope that this will get it through the test. If not, a pair of new cylinders and shoes would appear to be the best way to go.
dominic clancy

Dominic

I have just looked up the specification for our MOT test in the UK. There is only a balance specification on the steered wheels and this is 25% difference from the higher braked effort wheel. There is no balance spec on the rear wheels. Do you know what your test specification is? This is not to suggest you ought to get yours more balanced but my guess is we all have quite a variation side to side.
John Francis

Would it not be a good idea to also check the rubbur flexible hoses of the rear brakes? Maybe the one on the weak side has swollen inside restricting the flow of the fluid. Just a tought.

Frank
F Camilleri

That is the case John However if one rear brake was subsanially less than the other it would be a pretty fair bet that you would fail the test due to one side rear brake being below the limit of efficiency.

This is an intriguing question and I too would be stumped.
Robert (Bob) Midget Turbo

There are no flex hoses at the rear wheels of the MGA, only the one hose that connects from the battery carrier to the rear axle, and that cannot cause a difference from side to side.

My bet is a faulty slave cylinder. If you have time, swap the cylinders side to side. If that makes the problem switch sides, then you have a faulty cylinder. Pretty much impossible for a cylinder to put out more force then normal, so the lower force unit must be the bad one. You are probably in for replacing one (or both) slaves.

If that doesn't do it, the only thing left is a smashed or obstructed steel pipe between the 3-way fitting and the low force side. That could slow the flow enough to show low brake force on one side for a short period of time. Given a few seconds of time with foot on the pedal the pressure should equalize, but the braking test might be finished by then if the wheels stop turning that quick.
Barney Gaylord

Frank

There is only one hose on the back, id it supplies both brakes
dominic clancy

I take it all back. There is only one flexible rubber hose for the rear brakes and it serves both sides. But there could be a partial blockage inside the metal pipe. Worth a check.

Frank
F Camilleri

Well Dominic, you keep making it harder!
Does the dyno give you slip readouts between the wheel and the roller?

Note that in my above (last line) I got crossed up - it would be the RHR shoes you would file to reduce efficiency.

FRM
FR Millmore

Hi Dominic,Your problem is getting lots of suggestions - note that by simple physics , partial blockages in pipes will not affect the transfer of pressure and also the fluid does not "flow" as such. So my bet too is on the piston in the wheel cylinder being restricted in some way so it can only move a small amount and so only transmit a partial movement on the shoes. I would change both cylinders and not put new seals in the originals. I changed one of mine a few years ago and then found that the other side went shortly after. good luck - Cam
Cam Cunningham

All depends on how the dyno works; I've been looking but haven't turned up the info. If it has constantly driven rollers, then a steady state reading could be had, and Cam's point would be good. But that isn't really a good test of actual brake performance. If it spins up the rollers then measures how long it takes to stop or slow them a certain amount, and calculates force from that, then response time rules, as in the real world. And in that case, restrictions in pipes could have a big effect.

FRM
FR Millmore

From a basic principle perspective, the friction is dependent on the area of contact of the shoes on the drum and the load applied by the cylinders plus the relative friction between shoe and drum. Given that you have swopped the drums and shoes the surface friction characteristics of the shoe and drum can be eliminated. This leaves the load and the contact area. The load is dependent on the cylinders, x sectional area. I would reckon the pressure from the hydraulics will be the same at the cylinders and you say the handbrake shows the same characteristic so that is independent of hydraulic pressure.
That leaves the contact area of the shoe on the drum as a variable.
The shoe effectively rotates about the fixed post between the shoes opposite the cylinder and the ends of the shoe by the cylinder make the most contact with the drum. If the fixed post is slightly further away from the drum only the end of the shoe by the cylinder will make good contact. I'd measure the diameter of the shoes when at rest to compare the two sides of the car.
John Francis

John, the "fixed" end of the shoe is not pivoted, it can slide out to make contact.
Art Pearse

Dominic
Sorry I cannot remember the rear cylinder bore size for the MGA but I recall that the Morris Minor used a visually identical rear wheel brake cylinder but they came in two different bore sizes 7/8" and 3/4".
I recall from my MGA racing days fitting an Morris Minor wheel cylinder by mistake (it being incorrectly supplied by a local motor factor).
Just taking FRM's thoughts a step further.
Mark
Mark Dollimore

Dominic,

I still think you need to look at the cylinder bores for the answer to your problem.

If you look into the design of the cylinder the brake fluid only sees the first 1/4" or so of the cylinder. The remaining 1" or so contains the slotted piston and is not lubricated at all. I think you will find this area has some surface corrosion which is causing the cylinder to stick.

I would remove the brake shoes and pull the handbrake on to pop piston out of the outer part of the cylinder, not too far as you will make a mess of brake fluid.

Give the piston and the top of the cylinder a good clean and push it bask inside. It may well be fine then.

Paddy

P Reardon

Dominic

Taking up the theme of the possibility of an incorrect sized slave cylinder, I notice from your photos that they do not seem to be a matched pair. The right hand one, for example has a casting seam running down its length. Perhaps the bores are a slightly different diameter. As you put new seals on, perhaps the left side is slightly tighter causing it to move more slowly.

Steve
Steve Gyles

I think Steve has a point the 2 cylinders are not identical, that does not mean they are not the same internally but this problem need outside the box thought I feel.

As a note mine have the casting mark down the sides.

I thought T Series midgets also used this style of cylinder but what bore they were I have no idea?
Robert (Bob) Midget Turbo

Well I got home this evening and pulled the cylinders and inspected them. Neither shows signs of any wear at all in the bore, and there is absolutely no corrosion anywhere in sight despite being 22 years old. (Si brake fluid is wonderful stuff). The bores measured the same on diameter and depth with my digital calliper, and everything moves freely and cleanly.

I've put it all back together, slacked off the right side, tightened up the left, and bled so everything is firm. (Trust this to be the night that the eezibleed gave up the ghost, but it is twenty years old as well). Just as well I have great neighbours.

Here's hoping for tomorrow. I don't have access to the brake tester so I have no idea how the balance is now. I'll keep you posted.

Fletcher
The brake tester is a set of four rollers, one behind and in front of each wheel. The car sits in the V between them. It spin up the wheels so it measures if there is any prior resistance or drag. Then the brakes are applied to slow the rollers, so that it can measure the resistance applied by the brakes to measure the braking effect in Newton Metres. The machine has a cable remote that alows operation from the driver's seat, and two big displays to show the measurements of each roller pair
dominic clancy

Dominic. Looking in the workshop manual causes of braking imbalance include, greasy linings, distorted drums, tyres unevenly inflated, brake plate loose on the axle, different types of lining fitted, worn steering linkage, worn suspension linkage. The list includes some things we haven't thought of. Good luck tomorrow.

Art the shoes can only slide across the fixed pin radially looking at the manual and from memory, the shoe will tend to pivot on it since there is no force applied on that end of the shoe.


John Francis

John -
You are forgetting the self servo effects of the brake geometry, which is far to complex to get into here. Any geometry change will affect the servo action - condition and angle of pivot or reaction points, lining location and dimension, frictional characteristics of all moving surfaces great and small.

But, you may in fact have stumbled on a point. The fixed point <<should>> be radial to the drum, but: If the fixed abutment has dents, grooves, or an angle worn on it where the shoe rests, it can cause a very large change in the servo action. I do not recall how the abutment is made; typically it is a steel pad in that casting, usually hardened.

This can be problematic after a while, especially since one of the shoes usually has the (unused) notch for the Micram adjuster against that pad. 50 years of the shoe beating on the pad might be the problem. Wear here could cause either more or less servo effect, and possibly variable/reversing as the lining wears. And note that the actual wear or other fault of the abutment can be very small with very large effects.

This is also about the only part Dominic has not swapped.

Dominic -
Yes I figured all that, and saw pics of the setup, but no good description of how the actual data is obtained. Sounds like you are effectively reading application time as well as developed torque, so restrictions in pipes would detract from performance.

My brake dyno is a flat road and a heavy foot. Work up to rear lockup, then get out and look at the skid marks! Quite exciting when you have good enough brakes and rubber to actually lift both rears off the road.

Best of luck!

FRM
FR Millmore

Clearing the garage out today I came across a set of 4 new brake shoes I bought as spares for my Magnette about a year ago (same as MGA). Spotted that one had the lining cemented onto the shoe in a different position from the other three. (The odd one is on the left in the attached pic) This surely will affect the geometry and thus the braking force won't it?

Maybe worth checking Dominic.

Andy


Andrew Dear

Andy -
Yes,a definite fault, probably on the edge of being a real problem. I've seen much worse, to the point that the lining is completely toward the wrong end of the shoe. If you use them, use two matched ones as leading shoes, as that is the more critical position. On MGA/Mag rear, the leading shoes are the ones with the adjuster. In front they are all leading; you can cut back the one with the end too long, at some slight loss of braking. To be correct you would also cut back the other (trailing) end of the other shoe to match, but that is much less critical. Cutting back the leading end reduces servo effect, and also reduces sensitivity to variations of assorted sorts, making things more stable but increasing pedal pressure.

Note that the major effect of disc brakes is to eliminate all this fussy self servo business, in addition to better cooling. Cooling itself has major effects on the self servo state, since the coefficient of friction varies a lot with heat, or water, or grease, etc. The notoriously bad brakes of American cars in the 1950/60s was because the brakes were way undersized for the weight. A lot of it was to do with fitting 13 or 14" wheels with full covers to cars that weighed 4-5000 lbs, especially before vacuum boosters became standard. In order to stop it, they were made with a lot of self servo effect, and they would put you through the windscreen with light pedal pressure. But when they got hot and the friction dropped off, the servo effect left too and they could hardly be stopped at all.

Most important is that the two sides of the car are matched.
Dominic's pics seem to show that his are matched.

FRM
FR Millmore

Fletcher,
If you check around, you can find a used Clayton Brake dyno fairly cheaply. They used to sell around $50,000 if I recall. I haven't seen one since the late 80s. Electric motors on dyno and when you hit the brakes you show the braking torque.
Dominic,
It's possible that you are seeing a momentary high brake force on the one side due to line length or different line diameters to the back brakes. If you have aftermarket lines, you may wish to check this out. It doesn't take a large difference in inside diameter to affect fluid delivery.
Mike Parker

I have a serious problem accepting the value of the brake Dyno. I do not think that they give a result that is valid to compare with road use.
When braking a car on the road more than 99.9% (or more) of the braking effort is reducing the kinetic energy of a moving car weighing nearly a ton.
On the brake Dyno you are just reducing the rotational energy of the wheel and a few attached light parts.
The effort required to stop a rotating wheel is not sufficient for all the full self servo effect, temperature etc.... to come into the equation.
I use a Dyno to measure engine power at the wheels but not the braking of the car.
FRM's method of racing down the road is the best. You can feel any uneven braking that is sufficient to be a problem.

Mick
M F Anderson

Mike -
Well I'll just run out and get a couple, add to my piles of junk!
These things are used all over the place as test instruments, and do not show whacko results, so standard pipe configurations are not the problem. Fluid displacement is very small with correctly adjusted brakes. The difference in the MGA pipes on the rear axle is about 18" of what we hope is unrestricted tubing.

Mick - It is not that you are measuring torque to stop the wheel; you are measuring wheel/brake torque to stop the (rolling) road. That's why I asked if there was a roller vs wheel slip readout. The question is do they measure steady state load on the motor drive to the rollers, which is not a realistic brake test; or, do they initiate a signal and measure actual torque over a fixed interval or fixed speed reduction of the rollers? That would account for response and could easily be calibrated to simulate any desired load (vehicle weight, speed, hill, etc.) by modulating power delivered to the rollers plus inertia. Just as you vary load on an engine dyno. On a "rolling road" you are simulating moving the vehicle by moving the "road". I found a source where you could buy "weight charts" for brake testing older vehicles in the UK, so that is clearly an input.

FRM
FR Millmore

Dominic

I hope you passed the test. I have just got back from mine. Licensed to thrill for another year. Watched the test throughout. It was put on the brake dyno. Result was an improvement from last year, all thanks to the improved MC from AP Caparo - no slow release of pressure this time round.

Steve
Steve Gyles

Interesting that the UK MOT uses a brake Dyno for annual inspections.
Here we use a "g" meter placed on the floor on the passenger side for inspections.
The brake pedal force applied is measured in Newtons and the decceleration in "g".
The test is carried out on smooth dry concrete.
Image attached.

Mick


M F Anderson

Mick - That's the way it used to be done in UK about 30 years ago - the "g" meter was known as a "Tapley" meter if I remember correctly.
Cam Cunningham

Brings back memories Cam. I saw one in an antiques market a few weeks back! They wanted �40 for it.

One on Ebay at the moment for �85. I should have bought the one at the market - http://tinyurl.com/63v2y4b

Steve
Steve Gyles

Well the brakes passed - my guesstimate adjustment last night was enough to balance them out, but I will have to get to the root of the problem next winter.

Just have to fix the two fail points Front LH shock absorber obviously needs some fluid adding, and the headlights are pointed a shade too high after the accident repair. Both are easy to fix, will go and do now and go for a retest on Thursday morning.

Testing is a regional government-run thing here, and the main testing centre for the Canton is only 1km from my house, so there's not even any time for getting oil drips to upset them, or even burning themselves on anything hot.
dominic clancy

Way back I had an MOT on my old Rover and the guy used a brick on the floor as a g-meter. You had to roll it over on braking!
Art Pearse

Dominic

With your view on brakes and often advising us not to take chances, I am somewhat surprised you are leaving them to the end of the driving season.

Despite all you have done, I would have thought that for peace of mind you would simply buy and fit a new 'matched' pair of cylinders and see if that cures it. It is the only thing you seem not to have done.

Steve

PS. The only thing that did not work on my test was the windscreen washers (wire had detached from the switch - now sorted). However, the tester was not bothered. I guess he knew from the overall standard of the car that I would fix it straight away.

He also had trouble getting the horns to work, but he was trying to press the steering wheel boss!
Steve Gyles

Turns out the shock absorber on that side (which is about 8 years old) is shot. So I pulled the originals back out of the cellar and have put them on for the time being. I'll take the others and do a trade-in tomorrow.

Steve, I wasn't able to drive the car from July last year until about two weeks ago. it's either been being repaired from the accident or I haven't had time to sort out the new engine. I've been on it every evening for the last month more or less. I've fixed all the little things that have been irking me for ages, as well as redoing a lot of the reassembly the body place did wrong or used rubbish parts.

I'm going to get a new pair of cylinders and shoes, and I'll change it all on a rainy weekend, only takes 30 mins a side if you concentrate.

Just realised it only took three hours including getting changed, going downstairs to the garage, filling the installed shock, testing it, deciding it's duff and then hauling the originals out of the cellar, cleaning, painting and installing them, and getting everything put away and getting clean again. I've done things like this too often.

And all the trunnion seals were gum (rubbish rubber again), so I have replaced the top ones with ones I had in the cellar, and will do the others next winter.

dominic clancy

Fletcher,
I hope that the tubing would be unrestricted too, but you never know. BTW,Here is a brake testing dyno that I missed.
http://www.govdeals.com/index.cfm?fa=Main.Item&itemid=31&acctid=2706
Mike Parker

Passed on the retest today, but I am still not happy with the brakes, will be sorting very very soon with new cylinders
dominic clancy

Dominic

Well done. We will eagerly be awaiting the result of this saga. There will be lessons for us all I am sure.

As an aside, how is the rest of the rebuild? i.e. 5-speed, right angle drive etc.

Steve
Steve Gyles

This thread was discussed between 30/04/2011 and 06/05/2011

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