Hydraulic fluid question

[Bruce Parker (RIP)]

I've installed hydraulic clutch and accerlator systems in my old girl and need to bleed them. My question is regarding which fluid to use, regular or synthetic. I hear synthentic is better from a corrosion point of view especially if the cylinders won't be getting regular use...BUT...what if some of the rubber cups may have seen regular fluid some time previously? Some cylinders are new; some look new but 'smell' like regular fluid has been in them (as a lubricant on assembly perhaps). The guy who made my hoses says you can use synthetic in systems that have been filled with regular, but once you go synthetic, there's no going back. Can anyone confim this is true? Should I stick with regular fluid? The clutch master is in a particulary difficult place and I really really don't want to bugger it up.

[mike mckinley]

hi bruce

the only info i have that is close to your question involves hydraulic brakes. the u.s. military converted many of their vehicles from conventional brake fluid (dot 3) , to a silicon fluid. this was done for many reasons, one was that conventional fluid supports condensation, which in turn causes corrosion over time, leading to brake failure. another was that vehicles could be mothballed for several years because the fluid acted like a preservative in the system. their proceedure involved draining the system, flushing it out, and refilling with silicon fluid. what was found was that with older vehicles, the silicon caused the rubber components to break down and fail. with the newer vehicles, or ones that had their brake systems rebuilt in the recent past, there were fewer problems. this came down to the chemical make up of the rubber components. there is a u.s. army manual outlining the conversion proceedure, which i could e mail you if required. hope this helps

mike

[Bruce Parker (RIP)]

Thanks for your comments. The systems I'm working on are identical to brake hydraulics. My clutch is new, so silicone would be fine, but the accellerator (which shares the same reservoir) is of doubtful age and may be subject to the failures you mention. My brake system has new master and wheel cyinders, but the hydraulic/vacuum booster sat for 10 years with regular fluid. Oh what to do????

[Hanno Spoelstra]

Originally posted by Bruce Parker
the hydraulic/vacuum booster sat for 10 years with regular fluid. Oh what to do????

In that case, I'd stick with regular fluid.

Hanno

[mike mckinley]

hi bruce

as hanno suggested, going with the regular fluid is probably the safest bet. having said that, the manual i refered to was released in 1982, so i can only asume that's when the conversions were going on. when i made the reference to older vehicles i was thinking of first series m37's, m34's, and m151 / a1's, which may have survived without a rebuild since new. if your vac booster is only ten years old, it should work with silicon fluid, but in light of where it is located on the vehicle, and servicing requirements......the safe road is probably the best road.

mike

[Phil Waterman]

There is a potential leak problem if any of the brake system has sleeves. Silicon brake fluid will leak from a system that will hold regular fluid without leaking. This is according to White Post Restorations who has been doing brake cylinders sleeves for over 25 years. They have had a problems in the past with the silicon fluid leaking down the out side of the sleeves, so they advise against using it.

[Yeo.NT]

Do not use Synthetic in a Dot 3/Dot 4 system you will just end up making more work for yourself. Dot 3 & Dot 4 can be mixed Dot 5(synthetic) can't be mixed with Dot 3 or Dot 4. The synthetic will make the rubber seals swell and expand causing failure of your system.

[rob love]

There were some compatibility problems with the early synthetic brake fluid, which led to it's bad rap. We had some failures in the CF during our early conversions, which led to instructions as to which vehicles could get synthetic, and which could not.
Aparently, any synthetic fluid made after 1986 will be OK to use universally. Over the years, I saw many instances of guys putting the wrong fluid into the wrong vehicle, and never had the problems.

That said, I have never put synthetic into any of my vehicles, despite it's reputed superiority.

Might I suggest DOT 4, however, which is superior to the DOT 3 for it's heat applications. It saved my a** once, when I boiled the DOT 3 out of my old Ford while towing home my Ferret through the hills of Medicine Hat.

[Bruce Parker (RIP)]

Regular DOT 4 it is. The overheating is a concern as the accelerator slave is mounted on a bracket 3" from the exhaust manifold.

Thanks all for your help.

[Barry Churcher]

Good choice Bruce. Silicone fluid is not hygroscopic so will not hold moisture in suspension. What it will do though is separate out moisture and let it settle on the bottom of the cylinder pitting it worse. Silicone will hold air bubbles and they do not compress well, hence it is sometimes hard to get an adequate pedal. At the garage we will not put silicone in a system we have overhauled. I can put a guarantee on a Dot 3 system. For the antique seldom used vehicles we often work on we suggest bleeding the system each spring. This is an easy one man job with one of the vacuum bleeders and $3.00 worth of fluid.

Just my .02 CDN

Cheers,
Barry

[Bruce Parker (RIP)]

The process begins on the weekend, and all I have to worry about is whether the fittings leak...

You going to Caldwell's this weekend? Any luck digging out that 19 set stuff?

Originally posted by Barry Churcher
Good choice Bruce. Silicone fluid is not hygroscopic so will not hold moisture in suspension. What it will do though is separate out moisture and let it settle on the bottom of the cylinder pitting it worse. Silicone will hold air bubbles and they do not compress well, hence it is sometimes hard to get an adequate pedal. At the garage we will not put silicone in a system we have overhauled. I can put a guarantee on a Dot 3 system. For the antique seldom used vehicles we often work on we suggest bleeding the system each spring. This is an easy one man job with one of the vacuum bleeders and $3.00 worth of fluid.

Barry has stated almost exactly the warning statements well publicised by AP Lockheed that are on the net and copied verbatim below.

Further, AP L are forthright that they do not make or recommend silicone fluid and also state compressibility and lubrication problems with silicone fluid under tests.

It seems regular brake fluid is hydroscopic by design to entrain any condensed water throughout the system volume. Silicone fluids do not have this property and so water will settle, but worse than corrosion if a small droplet finds its way to a wheel cylinder or worse, a disc piston and then boils with conducted brake heat making a large volume of steam, the pedal goes to the floor and there are no brakes. Regular fluid will do this too when its hydroscopic capacity is finally exceeded.

Its noted that all vehicles now and for some years back have a fluid change schedule every 3 years to ensure entrained water levels cannot approach dangerous limits.

AP Lockheed had this to say:

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.

R.