Brake Fluid

Dave53-7C wrote:

Concerns were also expressed regarding silicones inability to absorb water. As such, water (if present) will boil and cause fluid to evaporate. As with anything else, take what you hear with a grain of salt and base your decision on personal experiences. Just keep your emergency brake in good repair.

Actually, evaporation would be good, but the water doesn’t go away that easily, the problem with silicone not absorbing water is that water is denser and therefore settles to the low point of the system. While the glycol based fluids absorb water resulting in a reduced boiling temperature but until that boil point is reached, the fluid remains a liquid and therefore “incompressible”. With the silicone you have an advantage of a higher boiling temperature, of the fluid itself. However, from the heat generated in the system, the pocket of water (conveniently settling at the heat source) boils at 212F and turns to compressible steam within the lines and cylinders, which can result in the spongy pedal due to steam in the lines (equivalent to air in the lines) as the system cools, the steam returns to water and returns to the low point. Though the master should allow the pressure to relieve and not apply the brakes for you, a poor cap vent will prevent this. By the way, if this happens, it will lock all 4 up solid until the pressure is relieved (not that I’ve tried it more than once, I didn’t like it enough the first time to do it again).

Another potential problem with silicone (Sorry Jon, like the switch, this won’t apply to you either.) that I’ve heard and it may just be an old mechanic’s tale, as I haven’t seen it through any credible sources. Is that with power brakes and a leak between the booster and master, fluid will be sucked through the vacuum line into the engine where the heat at pressure convert it to sand. Of course I’ve never heard any advantages of running DOT 3 through the engine either (other than to free a stuck engine).



10-15 years with silicone, I may have gotten 10–15 months at best for the hydraulic switch and silicone. I have been using the dot 4 or the synthetic for 10-15 years on the same switch though. Jon your car is just 50 or so years ahead of its time, as I had first heard of the problem with switches before my first failure, in an Old Cars Weekly article suggesting that you convert to a mechanical switch when using silicone due to the fluid’s toll on switches. The article was make specific and not Hudson realated, but the notes about silicone and hydraulic switches stuck with me. For the particular application, they actually had a motorcycle stoplight switch as a suggestion. When the first switch failed, from the article I at least understood the reason.