Silicone brake fluid queries

[Paul Drawmer]

In Demon Tweaks catalogue there is some silicone brake fluid.

1. it is DOT5 spec

2. it doesn't absorb water (ie not needing changes)

3. it doesn't harm paint

4. it's less money than other dot5 fluid ie AP Racing or Mintex racing

 

What gives, has anyone experience of silicone brake fluid and is it really a better fluid for less money?

(I tend to work on the assumption that if it looks too good to be true, then....)

[Rob Walker]

Do not be tempted to use Silicon fluid in your Seven. Sure it does not absorb water but moisture will get into your calipers and master cylinder, with conventional fluid the small amount of moisture will be absorbed into the fluid slowly degrading its boiling point. However with silicon the moisture can lurk in the calipers in small droplets and when you work the brakes hard these droplets will boil at 100c and expand rapidly into steam. Its not uncommon for total loss of braking and even small explosions using this suff. From my own experience the silicon fluid has a thicker viscosity than standard fluid and this makes bleeding the system even more difficult than normal. Its also very difficult to rid your system of silicon fluid once you have put it in. If you want a good high performance fluid try Castrol SRF its expensive but less hydroscopic than most other high performance fluids.

[F355GTS]

i use SRF in my 355 but am using AP600 in the 7 on Mr Grigsby's recommendation, it seems as good as SRF but a more reasonable £22 a litre compared to around £35 for SRF

 

Mark

[Bare]

Do not believe the nonsense about "silcone" brake fluid not damaging paint. Put some in my XK120 FHC for exactly that reason (car was a total restoration, and I didn't want any "paint Problems") BIG mistake.. It EATS paint, and while it does not absorb water.. it allows the unavoidable condensation to settle at the lowest points of the brake system. where it does the Most Damage... Lovely!

Don't forget abourt the well documented "incompatibility " with the Seals in the Brake system.. Sure some are "fine" but which ones?? Even brand new cars have proven "incompatible" Honda automatically Voids the Warrantee on anyone Stupid enough to use the stuff.

[Nigel Riches]

So if it's not hygroscopic how does the water get in the system to puddle and cause steam under heavy brake usage?, worried now, as I changed to silicone fluid years ago, 1984, I think, and have no problems with braking performance.

 

The fluid has been changed a few times, due to the rebuilds and general fiddling about, a further benefit is that as the small smear of fluid left behind when the brake pistons have advanced and retracted, actually help prevent piston seizure if the car is left standing for a prolonged period. Can't comment on the paint aspect, as mine's ali and glass fibre.

 

Just read another thread and apparently the water enters the system in suspension in air pressure that is felt over the exposed surface of the fluid in the master cylinder, if you follow, as the vibration moves the fluid surface the air is partially absorbed, giving up the water to the brake fluid, this is in the master cylinder, the water is heavier than the brake fluid and sinks to the bottom of the reservoir, eventually being compressed into the brake system high pressure side, but how does it migrate to the Hot brake components?, ie calipers, as the brake system is sealed (hopefully), there will be no movement of the fluid in the pipes, just increasing and decreasing pressure, I think.

 

Various threads on this subject don't encourage it's use for racing applications.

 

And apparently there are two D.O.T. 5 types of fluid, the other being D.O.T. 5.1, which is the regular Glycol based brake fluid, and not to be mixed with

silicone D.O.T.5.

 

For something so important there are some real traps for the unwary, strong feelings exist on this subject, be careful. Regards Nigel.

 

 

1982. 5 speed, clamshells. B.R.G / Ali. The True Colours.

[sjwb]

Actually what Rob described was exactly what happens to glycol based fluid *confused*; Never resulting in brake loss, but more ususally long and mushy pedal.

Use DOT 4 or 5.1 and change it REGULARLY.

 

Steve B

[Mike Bees]

I used the silicone stuff for a few years with no problems (no fade under track day conditions, certainly didn't have any effect where I spilt it on the 7s paint). Not a good idea to just fit & forget it though.

 

Mike

[Julian Thompson]

I also use silicone fluid and did end up with a slightly soggy pedal after many laps of Donnington (dry), but it was fine at Oulton (damp though) and Anglesy (dry),

 

Reckon the mad Castrol SRF is better but I've got no real complaints and my paint also doesnt seem to mind it. I wonder if cellulose is affected (XK120 perhaps) and 2 pack (Caterham) isnt? Just a thought.

[Peter Carmichael]

If you have previously used glycol brake fluids, you typically have to replace the seals throughout the brake system before using silicone. You can change back OK, because silicone does not affect the seals. The problem is with seals that have previously absorbed glycol and distorted.

 

Steve,

 

What Rob describes is correct and does not happen with glycol. He is describing liquid water in your calipers rather than a solution of water in the glycol - very different effects.

 

I have been getting problems with my hydraulic clutch which uses silicone fluid. The effect is like the clutch is being knocked back (particularly after vigorous use of engine revs). The clutch action comes back to normal after a single extra prod on the pedal. The clutch suppliers (Super Clutch) have told me that I am not the only one to report problems and suggest I move away from silicone ASAP. I only used it in the first place because I designed the master cylinder and I didn't trust it not to leak. It was exactly this condition that led me to missing a gear on my best run at at Curborough in August. This is a symptom I can drive around with the clutch but it would be a complete liability in a braking system. The fluid has not been proved guilty yet, but I will see if changing to a glycol fluid improves matters.

[Paul Drawmer]

Well, that was an interesting read..Thanks all for sharing your experiences.

 

In the light of your collective comments, and taking into account the fact that I'm chicken - I'll be using a convetional fluid then.