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albertamj

Ferret Brakes and Silicone Brake Fluid

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Hello, 

Those of you who have read my prior post regarding the restoration of 00CC78, know that I spent considerable time and money rebuilding the brake system on my Ferret.  The master cylinder was re-sleeved in stainless and everything else was rebuilt other than the metal brake lines - they were flushed thoroughly.  

The brakes never worked properly since the rebuild - they were soft and never built up pressure.  After a couple more bleeds, we dissembled the master cylinder, honed the stainless sleeve and  machined the piston thinking that it wasn't moving far enough to allow the fluid to pass from the cylinder into the system.  Still, no improvement.  Finally, we removed the silicone brake fluid, replaced the master cylinder cups and added dot 4.  Everything now works great.   

Epilogue - we suspect that the silicone brake fluid swelled the rubber cups in the master cylinder and it would get 'hung up' and not allow the fluid to flow through.  

Has anyone experienced any similar issues with silicone brake fluid?

Darcy

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I have always been advised never to use Silicon fluid in my Dodges. There was discussion  at the time wether this was relevant. . You seem to have answered the question.

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It has been suggested that it is much harder to bleed brakes when using silicon fluid as it tends to move around bubbles of air instead of pushing them along the pipe in front of it.  Presumably to do with surface tension. I think this is more of a problem with vehicles with larger diameter brake pipes than with modern cars that have 5mm OD pipes.

I think if the silicon fluid had swelled the cups they would either have not sealed or would have jammed. You seemed to be describing the feel of brakes that still needed bleeding (although you had bled them).

David

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Some years ago Lockheed issued a statement against the use of silicon brake fluid and they listed a number of issues that occur when using it. I recollect 'sponginess' of the pedal was one problem. I have never used it.

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The notice from Lockheed would be great to see. 

Others in our local MVPA club are restoring an M8 Greyhound with both hydraulic clutch and accelerator.  They too used silicone and are having nothing but grief.   That issue may well be related to David's comments as the diameter of the lines are quite large.

We even tried to power bleed the brakes on the Ferret with no noticeable difference.   

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44 minutes ago, albertamj said:

The notice from Lockheed would be great to see. 

 

I have not got the original notice from Lockheed to hand, but got the following transcription off another website:

Reproduction of a warning note from AP Lockheed on problems with silicone fluids in braking systems

Silicone brake fluids - beware
Our technical service department is receiving an alarming number of calls from motorists reporting problems with silicone fluids. AP Lockheed neither markets such fluids nor recommends their use with our own or any other braking system.

Virtually all the problems relate to long/spongy pedal, sudden loss of brakes and hanging on of brakes. They reflect certain properties of silicone fluids identified by us over many years and recently ratified in SAE publications, namely high ambient viscosity, high air absorption, high compressibility, low lubricity and immiscibility with water. Research has shown that the relationships between problems reported and properties identified may be expressed as follows:

Long/spongy pedal
o Compressibility, up to three times that of glycol based fluids.
o 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
o Air absorption - gasification of absorbed air at relatively low temperature produces vapour lock effect.
o 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 (100C or thereabouts).

Hanging-on of brakes
o Low lubricity - in disc brake systems the sole mechanism for normalisation if 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.
o High viscocity, exacerbating the above effect.
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 Supreme DOT 5.1, which exceeds the performance criteria of DOT5, is suitable for all conditions likely to be encountered in modern driving conditions.

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The subscript here is that you shouldn't use a silicon fluid in a system for which it is not specified. Given that brakes are a safety critical system, care should always be taken here.

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