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Ferret Brakes and Silicone Brake Fluid


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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?


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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).


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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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  • 2 months later...

I have been running silicone in my 101 Landrover, US6 Studebaker, modern work van,1960 willys CJ6 jeep for more than 10 years with no problems.

I find I get longer seal life , less corosion of brake parts and better lubrication of parts.

Silicone doesnt strip paint.

Never had a problem with bleeding brakes or a soft pedal.

Alcohol  based fluids tend not to lubricate and cause rust.

Most anti silicon negative reports are old ones based in the 1970s when silcone fluids were first beening developed.

The modern purple silcone fluid is made I think 3M or Dupont?...I dont think anyone else makes it regardless of the brand on the bottle besides the Chinese knock offs

A yellow chinese knock off is not worth buying.

If the brake seals are old new stock, or the system is hard to bleed and and silicone fluid is involved it naturally gets the blame.

Any water in the system will bleed though .

The presence of water in normal brake fluid as it asorbed into the fluid reduces its performance and why fluids should be bled though every two years

Any air in the system will bleed though cleanly with normal steady strokes of the brake pedal.

Both normal and silicone fluids should be bled though every two years regardless of useage.

Very happy silicone fluid user in all my vehicles except for the ones designed to use mineral oil in the brakes....ie Saladin, Stalwart ect.

What is not well known, but not recommended is small amounts of normal fluid or silicone fluid together will not cause any problems.

One other advantage of silicone is beening just that little bit thicker and slippery tends to seal wheel cylinder cups better.

The only draw back I found in real life if a wheel cylinder does leak with silicone fluid and the fluid gets on to the brake linings it is a reline job while with normal fluid can be washed off in water....................but the rule books say you shouldnt do this any how.


Edited by 101 Ron
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I was always under the impression that the american army were big users of Silicone flluid so presumably have not experienced any problems. 

I used it in my Humber Armoured Car and now in my Daimler so it will be interesting to see how it performs on their disc brakes.

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It is true that modern N. American military vehicles use silicone brake fluid - little yellow warning stickers everywhere advising the operator or REME tech of same.  From my own personal experiences with the Ferret, the entire brake system was replaced at great time and expense, so we thought why not go with silicone after coming all this way (for all the reasons that 101 Ron stated)?  A year of soft peddle, a dozen bleeds, removal and machining of the already rebuilt master cylinder and much teeth clenching, replacing the silicone with dot 4 made all the difference in the world.  The brakes bled beautifully, the peddle is firm and most importantly, the thing actually stops when I want it to!

I have since spoken to many 'experts' who seem to agree that if it didn't come with silicone, it shouldn't use silicone ...

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Hi all, I have just seen this and can fully empathise with those who have suffered problems with Silicone. I too have had vehicles converted over to silicone after thorough cleaning and flushing ( though silicone is advertised as being mixable) only to have terrible problems with master cylinder sticking. This was also the case, only worse, after stainless steel re-sleeving and I had to revert to the conventional brake fluid. I am relieved that it is not only me who has suffered such problems and I will not be tempted to use it again.


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Wondering if you guys in the UK are getting the Chinese knock off fluid.

The good stuff is purple in colour.

May add my 101 landrover has had disc brake conversion and re seleeved master cylinder, as have the clutch slave and master cyl……..again no problems.

( I may add 101 Landrovers suffer from transfer of brake fluid from one fluid reservoir to another  due to the design of the master cylinder and seals.....the silicone fluid stopped this problem due to its better sealing and lubrication properties) 

I have also used the stuff at work on different vehicles and on a 1995 four wheel disc braked defender 130 HCPU all with no problems after directly changing from normal fluid.

The US army I think has the world biggest truck fleet...……...I, don't think the US army would take on silicone fluid  if it was problematic or would anyone in the US supply it if there was the slightest chance of brake problems under the US consumer laws and litigation. 

Tests have shown alcohol based fluids do not store very well and loose much of the high temp properties after a year and Silicone fluids  don't have the same problems.

I wonder if the lower general weather temperatures you get in the UK is a factor as well.

Everyone will always have different experiences I guess.

Edited by 101 Ron
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6 hours ago, 101 Ron said:

The US army I think has the world biggest truck fleet...……...I, don't think the US army would take on silicone fluid  if it was problematic or would anyone in the US supply it if there was the slightest chance of brake problems under the US consumer laws and litigation. 

Silicon fluid is fantastic, in systems that are designed and specified for its use. If you are adapting or modifying a system, you'd best know what you're doing, although you can never go wrong following the manufacturer's specification.

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