This may sound a little strange, but one day a man called in looking for some water for his car. That was not unusual, until he started to put it into his hydraulic reservoir for the braking system of his car. I was young and curious, but he explained that water would suffice. Many years later and some hydraulic experience, it seems that he was right. Or partly so!
It should be remembered that twenty-five years ago, and anywhere from there back, the properties of rubber seals were different. Not deviating, but going some way to explain where the hydraulic oil/brake fluid, actually went. The fluid and indeed all oils were much different and had little agreement with the rubbers available at the time. Leakages were commonplace.
Though many may frown upon it, water is still efficient as a medium for hydraulic pressure, in the very short term, and especially in the case of a braking system in a general performance automobile. It is the purity of the water that changes its usability, but anything will do in an emergency. Incidentally, pure water is an insulator, though I wouldnt have the confidence to test the theory on a personal level, with four hundred volts. Lol! Seriously though, this would be a reason why battery top-ups, should be done with distilled or pure water. Water hasnt been simply, water, for some time, or since it was chemically analysed.
Modern hydraulic oils have anti-foaming, anti-wear (component-wise), anti-corrosive, lubrication, and heat dissipation properties. They also have special properties that limit their ability to compress, which is the area that hydraulic excels over pneumatic. Higher control comes with the solidity of the fluid, much as that sounds like a contradiction.
Indeed, pneumatic control is favoured where there are risks of contamination, from the oil itself, and this is one of the reasons that air is preferred to oil in a lot of production systems.
Friction does exist, even with fluid, and this as well as compressive forces, is the cause of heat generation. Any such heat generation, can result in a change in the liquids viscosity, thickness/thinness, ability to flow. It can also have a negative effect on the various sealing arrangement, whether steel on steel, rubber/plastic and its composites (sometimes brass or bronze impregnated polymers), or indeed, where cast iron replaces steel. Incidentally, dissimilar materials in juxtaposition and dynamically, are better than similar ones.
Steel on cast iron, generally, is better that steel on steel.
So, while this man went for the only resource available at that point in time, the pressure generated should be similar, or the difference negligible, he would have been missing the lubrication properties that oils achieve. His saving, was the short piston movement, the relatively slow speed of his vehicle, and the limited time of usage.
Hydraulics are exceptional at what they do, and large forces can be transmitted through tight radii, and indeed, returned through 180 degrees, such is its theory with respect to acting equally in all directions. Hydraulic systems have replaced the numerous and purely mechanical applications, where the moving parts are essentially and consistently lubricated. This is even better in a corrosive atmosphere, where for example, salty air will compromise all exposed parts.
On a lighter note, if such an improvising individual was thirsty, then he would have a choice between his brake fluid reservoir, his window washer reservoir, or indeed, his radiator.