Thread: Hydraulic oil as a replacment for clutch brake fluid.

Hi Circlework

While hydraulic oil would be fine from the point of corrosion control, lubricity, etc, the thermal expansion would cause issues. Brake fluid doesn't expand to any significance with rising temperature, whereas hydraulic oil will.

The consequences would be that as fluid heats up, you'll notice the free play in the brake & clutch pedals diminish. In fact, the brakes will start to drag as the fluid between master cylinder calipers/wheel cylinders expands...not good!

Regards
Brid

i was told once that it will swell the cups in the cyclinders

Heat isn't usually a problem on clutch cylinders. Silicone based fluids are available as a straight replacement if you want to get away from DOT types.

It does sound like a solution looking for a problem though. If you've got fluid leaking or needing topped up then the problem is the leak, not the fluid type.

I used to run Hydraulic oil in the power steering of my trucks and it cured the many leaks I had when using ATF, why not contact an oil company website or Rick-130 and get professional advice, Regards Frank.

Originally Posted by richard4u2

i was told once that it will swell the cups in the cyclinders

Sure will, tried some once as an emergency stopgap. Had to renew all the rubber cups and seals in the m/cyl. Bob

Conventional seals (and probably hoses) will be chemically attacked by hydraulic oils and will fail quite rapidly.

A good example of the problem is given by Citroens. Up until 1967 the very elaborate hydraulic system on Citroen cars used a special fluid (LHS2) that was effectively the same as heavy duty brake fluid. It was then replaced on new vehicles by a mineral based fluid, (LHM) that is very similar to an automatic transmission fluid. As a result, every rubber component in the system is different - look the same, but different, causing serious problems if the wrong part or the wrong fluid is used. (They tried to minimise this by a change in fluid colour from red to green, with matching changes for parts packaging, and painted bits changes from black to green.)

John

I was only talking about the clutch due to the high temps in the brake circuit.

OK, well that surely answers my question...... obviously the seals that are resistant to degradation by brake fluid, aren't to hydraulic oil.

And speaking of using hydraulic oil in the power steering, Ive been doing that for decades ..... Hyspin AWS46. But, I'm not recommending it, coz I'm no expert.

Thanks for your replies.


Daryl.

engine oil works fine, the rubber in the seals is the same as the rubber in the engine seals (usually)

but when its cold if you go too heavy you can get a very slow clutch return and it may even wind up dragging.

a monograde 10 or 15wt engine oil would probabley be your best hope if you want to go that way.