Bleeding silicone brake fluid

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.

Reply to
Mike G
Loading thread data ...

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!

Reply to
Willy Eckerslyke

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

Reply to
Jim Warren

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
Reply to
Rob

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.

Reply to
moray

I gave it almost 48 hours, and there is a distinct improvement. It's getting to the stage where "new shoes" explains most of the springiness. I'll give it another go later in the week, but I think it's just about MOT-able.

Here, by comparison, is how to bleed DS brakes. Attach tupe between caliper bleed nipple and LHM reservoir. Open nipples. Start engine. Apply brakes for ten seconds. Repeat for other caliper. Repeat for rear brakes using a can to collect the LHM. In practice, it's only the front ones which generally need doing. Total time ... ten minutes.

Sigh.

Ian

Reply to
Ian

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

Reply to
Ian

Ian gurgled happily, sounding much like they were saying:

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

Reply to
Adrian

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.

Reply to
moray

Modern?

Reply to
Ian Dalziel

MotorsForum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.