<VV> Re: BRAKE FAILURE

[Bill Elliott]

On Thu, 07 Jul 2005 13:35:51 -0400, Bill Elliott wrote:

>On Thu, 7 Jul 2005 10:20:15 -0700, Ron wrote:

>>You cannot feel any compressibility with DOT 5.  I think the talk about it 
>>is lots of hot air about not much.
>>RonH

>From the TR Register:
Bill
Silicon Brake Fluid 
A comment by Brian Smart, Service Technical, AP Lockheed 
We are often asked by members of classic car clubs similar to the TR Register, what are the benefits of Silicone brake fluid? 
AP Lockheed neither markets such fluids nor recommends use with our own or any other braking system, unless specifically 
designed to use such fluid. Virtually all problems relate to : 
-Long/spongy pedal 
-Sudden loss of brakes 
-Hanging of brakes 
They reflect certain properties of silicone fluids (advocated) by us over many 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/sponge 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 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 system [in] the sole mechanism for normalization of system 
pressure upon release of pedal pressure is a designed -in tendency of (the) seals to 
recover to their "at rest" attitude. Low lubricity works against this tendency. 
b) High viscosity, exacerbating 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 Lockheed glycol based fluids do not contain the adverse properties described above. The recently introduced 
SupremeDOT5.1 which exceeds the performance criteria of DOT 5 is suitable for all conditions likely to be encountered in 
modern driving conditions.