MOT fail - rear brake imbalance

[mikeh]

Hi all,

My TR4 failed its MOT yesterday for imbalance of the rear brakes – both the footbrake and handbrake were operating with reduced efficiency on the offside. Interestingly, this was certainly not noticeable in normal driving even once I knew it was there & was checking for it.

Having now taken a look, the cylinder in question is as dry as a bone, the piston moves freely and the brake shoes/drum are free from contamination. The only issue I could find was that the cylinder seemed somewhat reluctant to slide in its slot. I’ve therefore dismantled everything and have cleaned/regreased the contact areas so that it now slides more easily.

The fact that both the footbrake and the handbrake were below par would seem to point to the issue being mechanical rather than hydraulic, but I’d appreciate any other comments/advice forum users may have on the problem, including how I might check things before I take the car back for a re-test.

Thanks

Mike     

[Peter Cobbold]

I have had that on the 6, too thick paint and no grease and the cylinder fails to slide. A liberal coating of a mix of waxoyl and molygrease and vigorous frre-ing up has proved a permanent cure. Testing diy.....I never bothered but I suppose jacking up the rear wheels and driving them against the brakes should generate hot drums that could be measured.

Peter

[John Morrison]

Was the adjustment the same on both sides?

Also the cylinder can, over time 'Dig' into the backplater reducing of stopping movement,

do a search here but remedy is to change or weld up groove and gring back.

 

Do not doubt that you didn't notice a difference, but you might under an emegency braking situation.

John.

[Nigel Triumph]

I've had this on my GT6 a couple of times. First time, many years ago, it was due to wheel cylinder failing to slide easily on its backplate.

More recently, a similar problem was due to a seized wheel cylinder.

Nigel

[Z320]

I had this problem every two years at the TÜV though my brake is well serviced - until one TÜV guys told me:

"A brake is for braking! If you treat your beauty with to much care your brakes suffer!

And your front brakes are probably to strong and the rear has not enough to work!"

This is why with my last rear brake modification I changed the 0,70" cylinders to 0,75" (+15%),

had a fast run for 1 h on curvy and steep streets the weekend before TÜV - and everything was OK.

But before you should take car of everything else is OK with your brakes.

[John L]

Make sure the shoes are fitted the correct way round, to enable self energising characteristic.

Adjust the shoes with the handbrake cables disconnected, then reattach afterwards.

John 

[stuart]

Failure to slide is more often than not because the handbrake lever has dug a groove on the inside of the back plate where it sits, this needs to be welded up and ground back flat so the cylinder slides properly.

Stuart.

[BlueTR3A-5EKT]

14 hours ago, Z320 said:

I had this problem every two years at the TÜV though my brake is well serviced - until one TÜV guys told me:

"A brake is for braking! If you treat your beauty with to much care your brakes suffer!

And your front brakes are probably to strong and the rear has not enough to work!"

This is why with my last rear brake modification I changed the 0,70" cylinders to 0,75" (+15%),

had a fast run for 1 h on curvy and steep streets the weekend before TÜV - and everything was OK.

But before you should take car of everything else is OK with your brakes.

So you fitted what Triumph specified in TR3 and very late TR6. 
 

Rear brake imbalance 

Are the shoes clean, no oil or grease from the half shaft seals?

I have taken the cleaning the rear shoes and drums thoroughly before a brake test and telling the tester I have just renewed the shoes so they may be out of balance…….

The other trick was to tell the tester your car has a special limited slip differential and suggest they use a decelerometer.  https://aideautomotive.com/products/tapley-brake-meter-decelerometer/
 

Peter W

 

Edited June 17, 2021 by BlueTR3A-5EKT

[Z320]

3 hours ago, BlueTR3A-5EKT said:

So you fitted what Triumph specified in TR3 and very late TR6. 
Peter W

 

Yes I've been told this is the case,

so I did not worry about braking too much with this setting on the rear

[mikeh]

Thank you to everyone for their advice and suggestions. The good news is that the car passed its retest this morning with the tester saying that the brakes were excellent for an old 'un  - not sure whether he meant the car or the owner though?!

In the end, I stripped the mechanical parts of the brake assembly completely, cleaned everything and then reassembled, being careful to thoroughly grease the sliding surfaces. I didn't actually find anything of significance although, for good measure & comparison, I stripped the nearside too. The only point I did note was that the rubber seal around the cylinder fixing clips & handbrake lever was dry on the underside where it sits on the backplate. When the cylinder moves, this also needs to slide so I made sure that it was well-greased. Finally, to the point a couple of people raised about the handbrake lever creating a groove - it's clear where the lever pivots but I'd describe it as a slight indentation rather than a groove so, fortunately, it doesn't impede the operation and isn't at the stage of needing to be welded.

So, a good outcome in the end and I've certainly got a lot quicker at removing/refitting the brake shoes and the cylinder fixing clips.

Thanks again

Mike