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Silicone Brake Fluid in GMC ex Norway

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my 352 has developed some brake problems, I have at least one wheel binding and one not braking at all. No visible brake fluid loss, I guess the wheel cylinders at getting stuck. Now my question: the truck is ex Norway and has been converted to Silicone brake fluid. Should I keep it that way or return to normal Dot 4? Is there a risk of contamination of the whole system if I change to normal fluid and it mixes with remains of the silicone stuff? This is an early truck with the vacuum-boosted brake system so no hydrovac to be worried about.

Thank You for any help, Arthur

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Hi Arthur,


Silicone is much better for vehicles which might sit for longer periods of time as it isn't hygroscopic (doesn't absorb water). DoT 4 holds water and can apparently promote corrosion. Silicone (DoT 5) is not compatible with DoT 4 so you'd need to flush the system completely. However the rubber seals are specific for each type of fluid so if you decided to revert to DoT 4 you'd likely have to change every seal in the system. I'd stick with silicone: in your case it may be the cheaper and easier option :-). Just make sure your system is fluid-tight. I'd also recommend flushing the system with new fluid until fresh (purple) comes out of every nipple. This will remove any contamination which may have built up in the existing fluid.


I'm intrigued about your vacuum booster comment. the original vacuum booster IS a hydrovac (cylinder approx. 300mm dia, 500mm length; see picture).


- Mike

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Hi Mike,

thank You for Your most informative answer. What I meant with the vacuum booster was that the truck has that old booster, what ever it is, that is mechanically connected to the brake pedal. It seems to work, as long as it is that way, I don't touch it. Sticking to the Silicone fluid might be what I'll do, I do need new seals for the brake cylinders, so where could I start looking for the silicone-proof ones?


Greetings, Arthur

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  • 3 weeks later...
Sticking to the Silicone fluid might be what I'll do, I do need new seals for the brake cylinders, so where could I start looking for the silicone-proof ones?
Its your truck and you can use what you like.


AP Lockheed will not have anything to do with the stuff and say this:


"Our technical department receive an alarming number of calls from motorists reporting troubles with silicone brake fluid. 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, hanging on of brakes; they reflect certain properties of silicone fluid noticed by us over the years and recently ratified by SAE publications , namely :- high ambient viscosity, high air absorption, high compressibility, low lubricity, immiscibility with water."


You cannot mix the types but after flushing with methylated spirits you can use either separately and the seals are compatible with either type.


Silicone does not entrain water but will allow condensate to gather as drops causing corrosion, should this get to a wheel cylinder the boiling point is substantially lowered and may be dangerous. If the conducted heat from the shoe (but far worse with a disc piston) boils a drop of water you will have a gas (steam) and suddenly a pedal on the floor.


This is what killed Princess Grace of Monaco (Grace Kelly) before the days of understanding even with regular brake fluid, but of course when investigated, and the steam had recondensed by cooling, the vehicle brakes were found to be in order.


As you see AP Lockheed also cite viscosity, compressability, air entrainment and lack of lubrication.


Ordinary DOT 4 or 5 fluid is intended to entain water so there is never any large droplets to cause trouble, however, as you can see in any passenger car service schedule, the fluid should be replaced every 3 years to ensure the water retainment properties are not exceeded. I would think it wise to treat silicone fluid similarly and this somewhat negates the hype surrounding it.


Its a pity that diaphragms allowing volume changes without having the master cylinder reservoirs open to atmosphere are not used, modern motorbikes with hydraulic brakes (have to) have these and so do not suffer any water problems.


Your choice really as the opposing DOT 4 vs Silicone camps are very polarised.

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