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Is silicone brake fluid worth the extra $ ?

Started by Bob 13906, December 03, 2004, 10:03:14 PM

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Bob 13906

Im looking for feed back about brake fluids. Im rebuilding the complete brake system on my 47 (new ss lines, cylinders,etc.), and wonder if I should go with silicone or dot 3. I understand some of the differances,(dot 3 is hygroscopic and it eats paint). Thanks, Bob

Mike Delta

SILICONE BRAKE FLUIDS â€" A WORD OF CAUTION


AP RACING NEITHER MARKETS SUCH FLUIDS NOR RECOMMENDS THEIR USE WITH OUR OWN OR ANY OTHER BRAKING SYSTEM

Virtually all of the problems with Silicone Brake Fluids relate to : -

Long/spongy pedal
Sudden loss of brakes
Hanging on of brakes

They reflect certain properties of silicone fluids identified by us over may years and recently ratified in SAE publications, namely : -

High ambient viscosity
High air absorption
High compressibility
Low lubricity
Immiscibility with water

Research has shown that the relationships between problems reported and properties identified may be expressed as follows:-

Long /Spongy Pedal

a)   Compressibility, up to three times that of glycol based fluids
b)   High viscosity, twice that of glycol based fluids, leading to slow rates of fill and retention of free air entrapped during filling, and hence bleeding difficulties.

Sudden loss of brakes

a)   Air absorption. Gasification of absorbed air at relatively low temperature produces vapour lock effect.
b)   Immiscibility (failure to mix) with water. Whilst the presence of dissolved water will reduce the boiling point of glycol based fluids any free water entrapped in silicone-filled systems will boil and produce vapour lock at much lower temperatures (100°C or thereabouts)

Hanging on of brakes

a)   Low lubricity. In disc brake systems the sole mechanism for normalisation of system pressure upon release of pedal pressure is a designed-in tendency of seals to recover to their ‘at rest’ attitude. Low lubricity works against this tendency.
b)   High viscosity exacerbates the effect of (a) above.

It should not be assumed, therefore, that the high price of silicone fluids implies higher performance in hard driving or even normal road use.

AP Racing glycol based fluids do not contain the adverse properties described above. The recently introduced Formula DOT 5.1 which exceeds the performance criteria of DOT 5 (Silicone), is suitable for all conditions likely to be encountered in modern driving conditions.

Mike

As far as I can establish, it is very difficult to change from silicone to normal brake fluid.

The US Army researched it, but, I could not find the report.

Mike

Hey, that must be why they tell you to change your brake fluid every two years.

Bob Dziewit

I seem to recall reading somewhere in the 1957 factory literature (I cant remember if it was the Owners Manual, Shop Manual or an issue of the Serviceman) that Cadillac recommended changing the brake fluid on an annual basis.  So there must have been something to this.  Three of my vehicles have had DOT 5 for 25 years with no problems.

Brian Rachlin

I restored a 1936 Packard a few years ago.  The chassis was restored several years before the body was done, and the chassis sat around.  The brake system was completely new, all new lines, wheel cylinders and master cylinder.

When the rest of the car was done, I checked the brake fluid, and it was still full to the top, and the brakes operated flawlessly while I drove the car about 2 times a month for the next 3 years.

Since I never had a problem with the brakes, or the fluid, I would recommend using DOT 5 silicone brake fluid, but ONLY in cases where the entire system is new, or freshly rebuilt.  It does not interact well with DOT 3 old style brake fluid.

There is no practical way to flush out all of the fluid thouroughly enough to replace DOT 3 with DOT 5, so again, only use it if your system has been completely disassembled, lines flushed or replaced, and all new cylinders used.

DOT 3 is nasty stuff.  It will eat the paint right off if spilled, it absolutely does absorb moisture, gets cruddy, and is nasty when you bleed the system if you do a repair.  Why use it?

I can only go by my experiences.  I presently have a 1976 Eldorado that was very well maintained by the previous owner, and it has DOT 3, so I will have to stick with that, but I do intend on flushing it out over the winter and at least putting in fresh DOT 3.

When the stuff first came out, there was much debate about it, but I can only go by my own experience, which was quite good.

Regards,

Brian