Hydralic fluid and brake fluid, what is the difference?

[mopar]

Hmmm. It would appear to me that hydralic fluid and brake fluid do the same thing, transfer force to move a remote piston, so what is different? Since we buy hydralic fluids in 50 gallon drums on the farm, can we just use it instead of additionally buying brake fluid by the pint?

 

[brianl703]

I don't know what hydraulic fluid will do to a brake system, but adding motor oil or ATF to it will screw it up to the point that major repair work will be needed, by damaging the seals in the brake system.

Brake fluid is glycol ether. ATF/Motor Oil/Hydraulic fluids are mineral oils.

So I would expect that hydraulic fluid will have the same bad effects as putting ATF or motor oil into the brake system.

 

[labman]

NO, NO, NO!!!!!!!!!! Hydraulic brakes were developed before we had hydrocarbon resistant rubbers. They used poly glycols as fluid because they were too polar to be absorbed by natural rubber and swell it. Unfortunately it is hygroscopic, absorbing water and lowering its boiling point and making it corrosive. A problem, perhaps not as bad as many here fear, but a problem.

To this day, brake systems use cheap rubber that swells if you get a hydro carbon even near it. Obviously, the hydraulic systems have solved the problem. In fact some industrial brake systems do use hydraulic oil.

If you had a source for hydrocarbon resistant cups, you could rebuild your brake systems. For any brakes that get hard use, look at the boiling point too. I think higher boiling hydrocarbons may have higher viscosities. Don't even think about converting more complex systems such as a Hydrovac.

 

[mopar]

OK. Then what is different about the seals used in the brake system? Hmmm.

 

[mopar]

OK. When did hydrocarbon resistant rubber come into existance? Before 1968?

 

[labman]

I think W.W.II. If not, before 1950. The hydrocarbon, mineral oil, resistant rubbers use nitrogen, chlorine, sulfur, fluorine, and the ultimate, silicone to make them too polar to absorb hydrocarbons.

I am not sure what goes into brake parts, but most of them swell with the least bit of hydrocarbons. Seal conditioners contain polar materials to swell the hydrocarbon resistant rubbers.

 

[mopar]

And being from Manitoba where temperatures do dip below zero F I suppose the viscosity may be a real concern?

 

[mopar]

So what is the deal then? Are they only supplying use with type "A" seals that will only work with type "A" fluid and break down in type "B" fluid, or is there a legitimate reason that these two fluids are different?

 

[labman]

I think most of the hydrocarbon resistant rubbers resist brake fluid too. In fact, I have replaced the cup in my clutch slave cylinder with one out of a Hydroboost kit. (power steering fluid) Works fine and maybe outlasts the ones in the clutch kits.

They are different because they were developed in different eras, and you can't change now. That is what great grandpappy used.

 

[mopar]

Thanks all for the quick education.

 

[MolaKule]

Mopar,

Both fluids are hydraulic fluids in the sense they transfer force via pressures.

The major difference is that most brake fluids are made from synthesized glycol ether esters and "hydrualic" fluids are manf. from mineral oils and other synthesized fluids.

Each affects different seal materials in different ways and should never be interchanged.

 

[Big Jim]

The biggest reason that brake fluids evolved they way they did was because of their need to resist heat.

There are other factors too, but perhaps they could have been delt with. Among them are cold performance (oil too thick when cold) and sludge resistance.