Funny peculiar brake issue

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GeoffTE
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Funny peculiar brake issue

Post by GeoffTE » Sat Sep 15, 2018 9:22 am

I have a strange issue with the brakes on SAR. 99% of the time the brakes are very good and the pedal nice and firm, but occasionally - very occasionally - a get a looong brake travel which is most disconcerting to put it mildly. A second push restores the pedal to normal operation and the problem does not return for, perhaps, another couple of weeks or so. I've not been overly concerned to date as I've not had any emergency stops to worry about, but the frequency of occurrence seems to be increasing, maybe happening once a week or so. (It only occurred once on our recent 855 mile trip to Curbs, Anglesey and Liverpool) I'm getting more concerned now that this phenomenon will happen at a critical time, so need l need to investigate, but where to start?
The servo is a NOS Lockheed obtained from a member on here and the MC a new, original style one, obtained from a trader.
So....what could be happening? Why are the brakes normal for the vast majority of the time, but "fail" , not fully, just once in a blue moon? I'm assuming it might be a seal or valve somewhere, but a proper explanation I cannot come up with. I'm preparing to strip something and fit new seals, but I would like to have a likely explanation before I start.


Geoff. (5324)
SE5 (SAR), Lotus Elise S2, ( now sold :-( ), 1965 Lotus Elan S2, Ford Kuga 2 AWD
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heathsabre
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Funny peculiar brake issue

Post by heathsabre » Sat Sep 15, 2018 9:58 am

It could be brake pad knockback. Does it happen after reversing round a bend? Flex in the stub axle or play in the bearings can cause movement of the disk which pushes the pads away a little. At the next application the pad travel is extra long causing the symptom you describe. We had this with a Coupé many years a go and we learnt to live with it!


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Funny peculiar brake issue

Post by halfpenny » Sat Sep 15, 2018 10:08 am

Master cylinder main seal not sealing as quickly as it should. Maybe poor spec rubber. Try a new seal.



GeoffTE
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Funny peculiar brake issue

Post by GeoffTE » Sat Sep 15, 2018 11:08 am

All good suggestions, thank you, but can't understand why it's so infrequent, and it doesn't coincide with any particular action on my part. If it's a seal, which I'm not doubting, why doesn't it happen more often????


Geoff. (5324)
SE5 (SAR), Lotus Elise S2, ( now sold :-( ), 1965 Lotus Elan S2, Ford Kuga 2 AWD
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Roger Pennington
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Funny peculiar brake issue

Post by Roger Pennington » Sat Sep 15, 2018 12:08 pm

I'd echo Tony's pad knockback suggestion. That's an issue which can still occur even with the later cars, with beefier stub axles. It's especially common after sharp turns (like small roundabouts) or when manoeuvring (especially backwards). Worth checking the wheel bearing adjustment (but obviously don't be tempted to do them up too tight!)


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Funny peculiar brake issue

Post by peter freeman » Sat Sep 15, 2018 5:55 pm

MC piston not returning with the pedal ( sticking down ) until it is disturbed by a further pedal action.



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Joe.
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Funny peculiar brake issue

Post by Joe. » Sat Sep 15, 2018 7:43 pm

If you want to reduce pad knockback you can by a kit to reduce stub axle flex from revington TR.

https://www.revingtontr.com/product/rtr ... el-bearing

I've fitted a similar (not identical) kit to my 5a and the braking feel and pedal consitancy was noticably better. Of course you could by it and find it doesnt fix your issue...!



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Funny peculiar brake issue

Post by halfpenny » Sun Sep 16, 2018 12:24 pm

Interesting link Joe. Did you fit this to a the smaller stub axle? It looks like a reasonable way of increasing the stiffness provided that you remember that the hub nut is to be tight, not slack. Anybody else tried this?



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