Bleeding silicone brake fluid

Dear All,
I'm recommissioning my Herald at the moment. After a certain amount of swithering, I decided to go for silicone fluid in the brake and
clutch. The clutch, which I did first, was fine. However, the brakes have been a real pain. After a first session, with almost a litre of fluid through the system the pedal was still very springy. A second session a week later improved things a bit. A third session today shows no bubbles at all emerging, but there is still a fair bit of spring in the pedal.
However ... I also have new shows and pads, which always adds a bit of bounce on the Herald till things bed in
And ... I have read claims that silicone fluid is a bit more compressible than glycol
So ... should I be worried? Have folk here had problems bleeding silicone fluid? Experience of it giving more spring to the pedal? Shall I just take the car out for a drive - the MOT station is 20 miles away over twisting moorland roads - and hope that everything firms up a bit.
Incidentally, I know that I could switch to glycol, but my reasons for silicone are fairly good, and I'd like to stick with it unless it simply isn't going to be possible to get satisfactory performance.
Ian
PS New master cylinder, calipers, rear cylinders, hoses (Goodrich) and most pipes.
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Ian formulated the question :

Does it have a servo and you are testing it with the engine running?
If so they can feel a little soft and springy with the servo working, try it again without. Its to do with you applying much more pressure on the pedal than you would when actually driving.
--
Regards,
Harry (M1BYT) (L)
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wrote:

Make sure the shoes are adjusted properly, have you tried bleeding with the shoes backed off, then adjusting the shoes to the correct postion?
Also make sure that all seals in the system are happy with silicone fluid, silicone fluid is not reccomended for my Morris Minor because it is said it can damage the seals.
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wrote:

No servo - just a bog standard Herald system.
Ian
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Ian wrote:

I was about to say that'll be the same as my Vitesse then - unless yours is drums all round. I never had a problem with silicon and don't remember any great difficulties bleeding the system. But perhaps I was less sensitive to any sponginess than you are. I'd certainly recommend you persevere rather than switching back to normal brake fluid.
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I'm not convinced. Provided you change ordinary brake fluid regularly - perhaps every couple of years - the parts should have a long life. And changing the fluid and the subsequent bleeding forces you to examine the condition of the hoses and pipes.
--
*When the going gets tough, use duct tape

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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Dave Plowman (News) wrote:

But realistically, how many of us want to change the fluid that often? Now that he's bought the stuff, and the cost factor is gone, I can't see any reason not to stick with it. I'm certainly not convinced that the system will feel noticeably different once properly sorted, and am also dubious that the trouble he's been having so far is down to the type of fluid.

ISTR the OP said he'd fitted steel braided hoses, so inspecting _them_ won't tell him much anyway.
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wrote:

I'm with you. I really can't see the point of changing to a silicone based fluid. DOT 5.1 has many of the advantages of DOT 5 IMO, without any of the potential problems. AIUI DOT 5 is not compatible with glycol based brake fluid, so unless a glycol based system is completely purged of any traces of the old fluid, any remaining 'could' cause problems. There could also be a problem with the seals themselves on an older system, as apart from the incompatibility of the fluid itself, early seals were not chemically resistant to silicone based fluid, meaning all the seals had to be changed for ones that were. Having said that, AFAIK all later manufactured seals are compatible with both types of fluid, but if the seals have been replaced with NOS seals it could still be worth checking. Mike.
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Mike G wrote:

We keep hearing about the potential problems, but always from people who have never apparently used the stuff. I agree that it'd be a mistake to switch to it from conventional fluid, but on a rebuilt system where all the seals have been replaced, I really can't see why not. That's what I did with my Vitesse and it's been perfect. Cost-wise, what's an extra 15 quid when you're probably spending 100+ quid on parts anyway? And if it's really an issue, you'll save by not having to replace the fluid every few years anyway. Spongy pedal feel? Not in my experience.
Advantages - knowing your brake system will come apart when you want it to without revealing a mess of rusted calliper pistons, etc; No damage to paintwork from spills so your carefully restored bulkhead can stay looking that way.

Officially, it shouldn't happen as the DOT ratings insist that all fluids can be mixed. But, I agree that it's best avoided.

AIUI, seals are only prone to damage from silicon fluid if they've already been exposed to conventional brake fluid, so _unused_ old stock ones are OK.
The funny thing with all this, is that if we'd we always used silicon fluid, the huge disadvantages of glycol based fluid would see it used only by genuine racers and the Max Power brigade while the rest of us would avoid it like the plague. And newsgroups would be full of doom mongers telling everyone to stick to DOT5!
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Willy Eckerslyke wrote:

Nor mine "Slightly springy" is how I would describe it, but it is only noticeable if you regularly alternate between a car with silicone fluid and one with glycol fluid. If you only drive the DOT5 one, you stop noticing anything after the first few days and it always feels perfectly normal after that.
And let us not forget that the OP has *already* renewed his entire braking hydraulics and filled it with silicone, so changing back is not a realistic option. The merits of one over the other is only of academic interest to most. Whatever the system is filled with, it will stay filled with unless and until there is cause to do wholesale replacements.
Jim
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Willy Eckerslyke wrote:

If I assemble brake components I use a liquid silicon, and run a normal glycol brake fluid. - Calipers, master cylinders and rear cylinders.
So I have a mix to start.I have no problem with mixing the two.
Where does the problem start with the mixture and why does one have to ensure the integrity of the fluid, where does it start to fail and how?
r
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There is one major disadvantage with Silicon fluid though. It's not hygroscopic.
This means that any moisture that gets in to the system, will gather, and eventually work it's way to the lowest point in the system, which is typically the brake calipers/cylinders. Where it'll then sit, until the brake caliper/cylinder reaches the boiling point of water, then boil in to steam, giving you some nice brake fade until things cool down again. However, flushing the brake fluid out won't get rid of that water sitting in the bottom of the caliper/cylinder, as the bleed nipple is at the top.
Hygroscopicity is the main advantage of conventional brake fluids. It may sound like a major weakness, but the fact it absorbs water is an advantage. Rather than the above scenario, any moisture gets absorbed into the brake fluid, where it gradually lowers the boiling point of the entire system (unless of course the brake fluid becomes saturated), and by routine maintenance (ie changing the fluid every two years), the brake fluid boiling point should never drop below critical levels. Plus 90+% of any moisture should get flushed out with a fluid change.
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The same goes for LHM, and I'm not aware of any dreadful problems caused by moisture in Citroen (and other) systems using it.
Ian
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were saying:

Indeed. It's a bloody big ADVANTAGE, not a disadvantage.
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wrote:

Modern hydraulic power brake systems will still work with air in the system, albeit with a delay in the time taken for them to apply/release while any air/vapour is compressed/expanded. This is due to the fact the pump pressure acts directly on the braking system, and doesn't rely on a fixed displacement once the pedal is pressed.
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On Sun, 31 Aug 2008 11:30:53 +0100, "moray"

Modern?
--

Ian D

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Ian wrote:

Personally I don't like silicon fluid not worth the expense. If you correctly flush your fluid every two years you should have no problems, except wear and tear on the system components, which is normal what ever the fluid you use.
Ok having had my rant, make sure that the rear brake shoes are adjusted fully up, this can induce soft brakes, these should also be adjusted up after the linings bed in.
After the system has been bleed let the car sit and check the pedal next day. You may check the play in the rod on the pedal and adjust that up as well.
r
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Like I said, it was a close call, and if all else fails I'll go back to glycol.

They're as well adjusted as any new shoes in new drums can be - I'll certainly expect to re-adjust them after everything has bedded in.

No adjustment at the pedal or in the cylinder. However, leaving the car to sit for a few days after the first bleed did seem to improve things a bit - mainly, I think, by allowing microbubbles to coalesce into bigger, bleedable lumps of air.
I've done some limited road testing and the brakes work OK, without pulling - it's just that the pedal feels bouncier than I'd like. And, I fear, than the MOT tester will like ...
Ian
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Ian wrote:

If the problem is micro bubbles, then the answer we found (on motorcycles) is to pressurise the system and leave it pressurised over night, the reduction in pedal travel the next day is quite amazing.
I use a brake pedal jack (home made from a mastic gun) to hold the pedal down. Also useful for checking brake equality and brake lights !!
Mrcheerful
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Splendid idea. I shall investigate a bodge along those lines!
Ian
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