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Safety of our hydraulic brakes?

Mike R

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This has come up actually on my clutch, but the same could happen on the brake system which worries me a little.

I check the fluid levels regularly, and both clutch and brake levels were fine a couple of weeks ago.

I did a 300 mile round trip without problem.

I then came to use my car for work on my regular "fun car Friday" (I usually cycle) and no clutch.

On investigation there was no fluid in the clutch master cylinder.


I found fluid under the master cylinder gaiter and on dissassembly found the remaining fluid to be a mucky grey as if the seal had decomposed into the fluid. Yet the seal looked fine. This is a seal i fitted a year or so ago, so could be poor quality part?

Does anyone have experience of failed seal in this way?


What worries me is if same failure occurs on the brakes when on the motorway the consequence could be shall we say not good.


So to reduce this risk I was thinking of fitting a low level warning to the brake master cylinder.

By careful use of relays think i can use the demister warning light, so that it flashes on low level, and yet still use it for its intended use.


Also by powering the level switch off the IND terminal of the alternator to energise a relay, the system will self test when ignition is on without the engine running. When the ignition is turned before start, power comes onto the flash circuit causing the bulb to flash. It is only when the engine is started that power comes from the IND terminal and pulls the relay in and turns the flashing light off. If the float goes low the relay de-energises and we get flashing again.


Any thoughts or experience on doing this sort of thing?



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I don't see why a float wouldn't work. The electronics look complicated, I would just add an extra warning lamp somewhere.

And learn to check things regularly, especially so before a long journey


lastly, if the cylinder bore is poor, then it can cause problems with the seals. I tend to use 800 wet/dry paper with a bit of brake fluid as lubricant on a bit of wooden dowel (makes a flapwheel sort of thing) to hone the bores, and that does seem to work. (a dozen or more cylinders over the years)

You may need to source a new cylinder if seals are not lasting. Unless we now have dodgy seals in the supply chain.....

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Hi mrantell,


I think you make a good point.  I think it is a good idea to have a brake-fluid warning-light on our cars, particularly as most employ a single-circuit system.


I have a similar piece of kit on my car (a Vitesse).  I have a handbrake warning-light (from a 60's Jag) on the dashboard in place of the old ignition switch (as converted to steering-lock) which doubles as both handbrake-warning and to monitor low fluid-llevel.


To make the handbrake warning-light, I adapted something from a modern car (I expect most will do; mine was from a BMW e36), and for the reservoir-level, I used a Smiths aftermarket device that fits into the reservoir lid.  There's a little button on the top that is used for testing.  Your system sounds a little more sophisticated.  But in any event, an early warning system for brake-fluid loss has got to be a good idea.





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Thanks for the replies.

I checked the bore and it seamed fine. But i will now see if i can hone it. I've replaced.with a new cylinder for now, but i'd like to replace the original girling unit eventually.


The complexity of the electronics was for 2 reasons.


Firstly i wanted to reuse the demister light rather than have something adhoc - but still use it for its intended purpose.

Secondly, i'm not sure I would notice a little light coming on quietly in the middle of a journey. Whereas something that flashes gets the attention.

But then i suppose simple often works best!

The one thing that worries me is sealing the floatswitch into the cylinder cap, it would be ironic to cause the cylinder to lose itse fluid by making this mod.

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Not sure if another cap would fit, have a trawl through ebay, usually turns up odd stuff that can be useful. Must be something from teh late 70's onwards that shares similar MC?

Re warning lamps, there are some VERY bright and small LEDs about (just wired one in with a pair of diodes so my Toledo indicator WL just has one light now, much brighter than teh std bulb so will need attention at some point.


Dual circuit MC's are available (sometimes!) for GT6, but work if one circuit fails. If you lose all your fluid you are still stuffed. But they are a good idea. Of use a pair of MC's with a balance bar?

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Dual circuit master cylinder is definitely a good solution. I'd personally rather leave as original as possible. I know I'll have an odd switch sticking ouut the top of the master cylinder, but that's easy to reverse if needed.

I like the idea of ultra bright LED - that might be a simpler way of ensuring its noticed rather thgan my rather complicated design. I'll think about that one.

On the cap, I've bought a switch designed to be retrofitted to an existing cap - see picture in original post. So bought a spare cap to modify and fit it to. Reverting to original is then just a case of swapping caps.

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I fear that when we own and run such old cars, it is incumbent on us to maintain them properly. 


"Motorists" used to be exhorted to check their tyre pressures every time they went out, to check the fluids weekly and grease the suspension every few months.    Modern cars are not only a light year more reliable, but they check themselves!     I have a slightly less that perfectly sealing tyre on one wheel of my modern, and every few months it tells me - low pressure!     In fact, the pressure is less than 5 psi lower than ideal, and it gets pumped up.


We cannot let that extraordinary reliability make us forget that our cars are from that previous era.  As you say, yours was a clutch fluid leak, not brakes, which could be rather more embarrassing.   A quick check under the bonnet and a critical look at the tyres should be a rule before each trip, IMHO.   I really don't think that this was a sudden failure, instead one that you would have noted on a journey and taken the right action, even if it stranded you, but regular checks can reveal a problem long before it become critical.



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I agree wirh everything you say John and I do check my fluids regularly, just not every trip. In this case the check had been something like a week earlier with as normal no issue - and only a short 3 mile journey to work and back in between the check and the longer journey. The difference here was that I had just completed that long journey, I would think whilst only doing short journeys the clutch doesn't get cycled that many times, then go on the long one it does.


What worries me is that it seams weekly check is not enough. I guess a small weep might have been noticeable under the gaiter if I had checked there. But I wouldn't normally check there. If I had checked whilst parked up at the inlaws I might have spotted something. But this was for me a sudden failure, repeated checks of a full cylinder, then empty less than 400miles and 1 week after the last full check.


So I'm treating it as a bit of a near miss, how can I reduce the risk without impacting my enjoyment of the car!

Hence the idea of fitting a switch to the brake master cylinder - at least would be able to bring the car to a halt before a complete failure then. Not worried about the clutch as worst thing then is that it leaves me stranded.

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Well thanks all for the comments.

I've thought it through and concluded that my clutch hydraulics must have an issue - probably the bore of the old master cylinder deteriorating the seal, so hopefully the new cylinder has sorted that. Whereas my brake master cylinder hasn't needed topping up in the 16 years I've owned the car. So I think the brakes are fairly low risk. Based on that have decided not to go with the complicated circuit originally considered.

I had already ordered the float switch and buzzer as a test so thought i'd take a look and see if its worth instslling something or not.

Turns up, installing a simple buzzer based alarm is very simple. Float switch fits really easily into spare master cylinder cap, power comes from the green cable connected to the windscreenwiper switch straight to the buzzer, through the bulkhead to the new float switch on he brake cylinder and then to earth next to the wiper motor.

So my little safety improvement is now installed. Probably not really needed, but just a little extra protection. I'll still check the fluid levels though.

I'm taking the GT6 to LeMans so gives me more confidence that the brakes will keep me safe!




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If you have not needed a top up in 16years I hope you have changed the fluid, two years is the max life

or you will have more moisture in the reservoir than fluid,


in the old days many manufacturers linked a simple cap switch to operate the interior lamp


No one read the handbooks and could not work out why the int light kept flashing on bumpy roads



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Yes brake fluid has been changed - usually because of other things needed doing. Although must admit, maybe claiming never topped up might be a slight exageration, certainly can't remember having to do it when checking though.


But ... Now i'm not too sure about my "safety" mod.

I knew i was getting rid of the little breather hole in the middle of the cap. But thought the need for the brake cylinder to breath would be minimal, with very little actual movement of fluid when you press the brakes. Also, likelyhood of the cap being sealed well enough to allow vacuum to be pulled seams unlikely.


But I did a little searching online and found some info where a different type of cap could get its breather blocked and result in erratic brakes. The main requirement for breathing according to this article being expansion of the fluid as it gets hot.


Seeing as its brakes, and not wanting to take any risks, i'm now going to reverse the mod and put the old cap back on. At least until I can confirm whether this is an issue or not.

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The level changes as you apply the brakes, pop the cap press pedal, should see some level movement as you press


also if its sealed as the air in the resv.heats it expands and will pressure the brake line probably more than fluid growth

Keep the breather


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