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Mot failed, very glad too


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My TR6 failed it's mot today with a dodgy rear near side brake. It was about 50% as effective as the offside brake. So why am I glad, well the car showed no braking problems on the road under normal driving conditions but I would imagine that if I needed to perform an emergency stop then the imbalance could have had a destabilising effect prcisely at the time I would be looking for maximum control.

 

Investigating the problem I've found a partially blocked rear flexi pipe. What a sod that proved to get off, still it's off now and I'll be getting a replacement tomorrow and hopefully a successful retest will follow..

 

Gary

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My TR6 failed it's mot today with a dodgy rear near side brake. It was about 50% as effective as the offside brake. So why am I glad, well the car showed no braking problems on the road under normal driving conditions but I would imagine that if I needed to perform an emergency stop then the imbalance could have had a destabilising effect prcisely at the time I would be looking for maximum control.

 

 

And pre 1960 cars don't need MOT's........................................................................ :ph34r:

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Look at it this way, there'll soon enough be plenty of pre-1960 cars without MoTs writing themselves off . . . . and enhancing the value of those that remain. :P

 

Pity about the poor sods who get in the way of a defective old nail en route to self destruction, guess we'll just have to call them collateral damage . . . .

 

Cheers

 

Alec

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well today went badly.....

 

got up and drove to paddocks in Chester, about 40 mins, decided to do a belt, braces and overcoat approach and bought two hoses (rubber), two slave cylinders and just in case a new master cylinder (use or return basis).

 

fitted the offending side slave cylinder but discovered the new rubber hose had different thread than the old one, naturally after I'd wrecked the thread on both the new hose and old pipe union. So back to Chester for another hose and new brake pipe. Now 1.00pm.

 

everything fitted by 1.30 but unable to get fluid through pipe. Conclusion?, stuffed master cylinder due to pumping. Fitted replacement new MC and eventually got all the air out of the system.

 

By 3.30 it was back to the MOT tester with fresh brakes and confidence. result?

 

Failed again, same problem identical reading!

 

 

aaaggghhhhh

 

even the tester is baffled.

 

Handbrake readings on both rear wheels are very good so shoes and drums are OK and well adjusted (about 150 whatevers)

 

Individual readings (taken one wheel at a time) are both strong (about 210 whatevers) so pressure to each cylinder is good

 

Balance readings are terrible (both wheels at the same time) with the left wheel about 80 and right over 200 (whatevers)

 

I've run out of available time to fiddle so I've done the unthinkable and for the first time in 20 plus years of fiddling with old cars I'm paying him to sort it out .

 

I'm shamed!!!!

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We dont know that the same force was applied to the pedal in the individual tests. If the weak cylinder slides with resistance in the back plate and the good cylinder slides freely that could give different readings in the dual test. The handbrake test may overcome the resistance to sliding on both.

The difference is too big to be explained by different bore cylinders, but there are two sizes available.

Peter

Edited by Peter Cobbold
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thats a good point but the 'test' was repeated 8-10 times with similar results every time. The exact numbers varied but were always similarly proportioned, i.e.. Individual testing shows approx parity between sides and simultaneous testing showed approx 2:1 bias in favour of the offside brake.

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i had a problem before anything was changed and the same problem after the parts were replaced. if there was still air in the underperforming side it would underperform when be tested at all times not just during the balance test.

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Clearly that's the case here but the flexi pipe to the weak wheel was blocked ( can't even get air through it) so a new flexi pipe, copper pipe and cylinder would seem to be a good idea wouldn't you agree? Clearly the is another problem that was there initially or I've created in replacing the inoperative parts. Either way the symptoms are identical and that's either coincidence or something yet unchanged.

 

We'll see when I get it back later today hopefully fixed.

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Hi Gary,

did the original flex pipe go duff by disintegrating on the inner bore.

It could be that some old rubber has got into the pipework to the inlet of the wheel cylinder thus blocking the entrance to fluid.

 

Remove the flex pipe and disconnect solid pipe from WC and blow through solid pipe.

 

Does fluid come out of the flex pipe when disconnected.

 

I would have though air anywhere in the system would cause all brakes to become less efficient.

As pressure builds up anywhere the air will compress.

 

Roger

Edited by RogerH
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.

 

I would have though air anywhere in the system would cause all brakes to become less efficient.

As pressure builds up anywhere the air will compress.

 

Roger

That's what I thought, and so doubted Neil's comment about an air bubble. However I see that Pascal's law only applies to static pressure. If the pressure at the cylinder fluctuates due to say an oval drum ( see Kev's post above) or maybe even normal stick/slip behaviour of the friction linings the volume of fluid in the cylider could rapidly fluctuate with the pressure changes being taken up by the bubble rapidly changing volume. And leaving pressure on the good cylinder unaffected. So I think an air bubble can act as a sort of shock absorber and so explain the asymmetry in the duel test.

Hopefully a hydraulics expert will comment.....

Peter

Edited by Peter Cobbold
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well its all sorted, both sides approx equal at 119 and 122.

 

so what did he do?

 

changed the shoes 'but the old ones looked OK and weren't contaminated', rebled the brakes 'but no more air came out' and adjusted the brakes with the handbrake cable disconnected 'connected and adjusted the cable after '

 

he's still a bit mystified but the readings are good and the MOT is mine

 

happy motoring again

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Old foot pumping method

 

Have to say that I'm cautious that the 'feel' is very different with more pedal travel that earlier. Pedal firm and brakes obviously good but the pedal difference may simply be due to the new master cylinder.

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Old foot pumping method

 

Have to say that I'm cautious that the 'feel' is very different with more pedal travel that earlier. Pedal firm and brakes obviously good but the pedal difference may simply be due to the new master cylinder.

When I fitted new m/c the pedal felt identical. I would readjust rear brakes, with h/b cbale in place, after a few hard brakings to centre and bed in the shoes.

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Might be interesting to take a look at the shoes that came off. I've had a couple of incidences lately where shoes sets have been supplied with all 4 having the friction material glued in the same position. On saloons at least (and I suspect TRs are the same) you should get two pairs. While the shoe structure itself is the same, the friction material is fitted differently and sits at different ends of the curved section. Fitting two identical rather than a pair might well affect the self-servoing action and thus the braking effort. Of course, in your case it would have have to have been different on each wheel.....

 

Nick

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