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Imbalanced rear brakes: causes and cures?


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Peter

With respect,he needs to find a quite piece of road let go of the steering wheel and hit the brake pedal,the rollers do not lie

 

Already done, and no appreciable pull one way or t'other...

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Peter

With respect,he needs to find a quite piece of road let go of the steering wheel and hit the brake pedal,the rollers do not lie

I agree completely.

Nothing more scary than one rear wheel locking and launching you sideways.

Peter W

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You won't lock a rear wheel. That only happens if the front brake/s fail; however, an underperforming rear brake will cause the front on that side to lock up. Everbody then rushes to find the "fault" in that front brake.

Just for interest, & in view of the mystery surrounding this car's imbalance:

AFAIK Girling rear cylinders all appear identical outwardly- there were three bore sizes specified at different points in TR production, 0.625", 0.7", and 0.75". Each of these fits in the same fashion- the next time the brakes are apart, simply pull off the piston dust covers, and ease the piston out enough to check its diameter. If there is a discrepancy, the side showing the least braking effort will have the smaller cylinder.

Oi'll git me anorak..

SPMPW

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You won't lock a rear wheel. That only happens if the front brake/s fail; however, an underperforming rear brake will cause the front on that side to lock up. Everbody then rushes to find the "fault" in that front brake.

Just for interest, & in view of the mystery surrounding this car's imbalance:

AFAIK Girling rear cylinders all appear identical outwardly- there were three bore sizes specified at different points in TR production, 0.625", 0.7", and 0.75". Each of these fits in the same fashion- the next time the brakes are apart, simply pull off the piston dust covers, and ease the piston out enough to check its diameter. If there is a discrepancy, the side showing the least braking effort will have the smaller cylinder.

Oi'll git me anorak..

SPMPW

One of my enduring memories is of the rear wheels locking in the wet on my TR3A. Changing to 3/4 cylinders from the 5/8 improved things no end - as recommended by one Pete Buckles (who sold the things) Did any one else suffer this issue?

 

Peter W

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Malcolm

Interesting, because I took a 1969 Morris Minor for an MOT saturday. As usual the balance was way out but the tester explained that the requirements had recently been relaxed and now virtually ALL old cars he has in pass first time, previously around 50% failure.

So I came away with a certificate.

Also remember that if you have the drums machined you will need to "run" them in, the diameters now being different. I did this very recently on another Minor and there was virtually nothing initially, improving with use. The benefit of the pre '60 no test allowed me to run them in for a few miles.

Colin

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Sorry, wrong again. You'd have changed from 0.75" to 0.625" to decrease braking effort, especially if you were on 10" rears. Front disc brakes would be cold & possibly damp in the conditions, too.

Agreed Jon

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Sorry, wrong again. You'd have changed from 0.75" to 0.625" to decrease braking effort, especially if you were on 10" rears. Front disc brakes would be cold & possibly damp in the conditions, too.

Happy to be corrected.

I agree the discs would have been damp at the point of brake application on a wet road. There again it may have been the 'White Box' brake pads. It all worked acceptably in the dry at motorway speeds, unlike the previous car a TR2 with drums, where at 70 mph it felt like you were pulling on a pair of jeans with the leg bottoms sewn up!

 

The rears were 10 x 2 1/4 standard arrangement.

 

In truth I have no idea whether the increase in rear cyl piston diameter increases or decreases shoe pressure for the given pedal pressure. Changing both the rear cylinders solved my locking up issue - may have been down to basic lack of maintenance of course.

 

No doubt a 'fluidist' will be along in a moment to put us all right..

 

My car now has .7 rear cyls (with 10" x 2.25" drums) and TR 6 front caliper/disc set up so the balance is probably now all over the place - especially as we have no control what so ever what friction material we are able to buy without extensive expense/testing and matching of disc pads to the available rear shoe material. Unlike when the car was in production and a standard range of products was provided by the original brake system specifier.

 

Cheers

Peter W

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In truth I have no idea whether the increase in rear cyl piston diameter increases or decreases shoe pressure for the given pedal pressure. Changing both the rear cylinders solved my locking up issue - may have been down to basic lack of maintenance of course.

 

No doubt a 'fluidist' will be along in a moment to put us all right..

 

 

I'd guess that based on Pressure = Force / Area, and hence Force = Pressure x Area, you'd get more breaking force with the larger piston area for the same pedal effort... but my high school logic very often proves to be an over simplification so I expect to be corrected shortly :)

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If there's a driver pushing the brake pedal (and it's difficult to imagine how else the test could be conducted!), then the braking force measured at the rear will be greater on the side on which the driver is seated purely because the driver adds to the downward force on that side. With a passenger on board as well, the measured force should be nearly equal (unless the passenger is hugely different in weight to the driver!).

 

To decrease braking force, reduce the size of the piston in the slave cylinder(s).

 

To increase braking force to all 4 brakes (i.e. front & rear), decrease the diameter of the master cylinder's piston - but brake travel will increase proportonately.

 

Ian Cornish

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  • 3 weeks later...

What a coincidence, same day same problem except that mine is a '58 car therefore it was a 'non MoT' and it did meet the new requirements but only just.

 

Looking at the top shoe on RHS it is clear that only the front and rear of the lining has been contacting the drum. Haven't done any more yet as it is p*ssing down here.

 

Update on this. I swapped the shoes from side to side then took it the 400 yards to the MoT station and put it on the rollers. Rear brakes now excellent rather than just OK.

 

It pays to try the simple things first.

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