kstyer said: At least in the past, the DOT 5 did have a lower boiling point. The spec Steve stated sound odd. When I go into the next parts store it's time to read some updated labels. Silicone is NEVER compatiable with regular brake fluid.

maybe it means you can use it in dot 3 and 4 applications..

you obviously cannot mix them

Brake fluid boiling point is a very good comparison for evaluating regular or polyglycolether based fluids, "glycol" for short, because it roughly represents the conditions under which glycol turns compressible. On the other hand, it is a very poor comparison for evaluating silicone based fluids because they turn compressible at a lower temperature than that at which they boil.

To add to the confusion, (presuming a properly bled system with negligible air bubbles), non-silicon fluids get compressible only after they boil (with absorbed moisture lowering this temperature), however the silicon fluid is four times more compressible at elevated temps (but before boiling point is reached). This is also not helped by the susceptibility to micro-aeration. This is the crux of the problem when discussing the fluid options.

Glycol based brake fluid is hygroscopic, i.e., it absorbs water from the surrounding atmosphere, and it starts adsorbing water as soon as you put it into your car's brake system. The brake fluid reservoirs on top of the master cylinder are vented to the atmosphere. Moisture can also enter the brake system through the rubber seals and hoses through a 'diffusion' process

from what I have found by looking around this evening..

you use dot 4 or dot 5.1 (which is much more similar to 4 than 5) for a frequently driven performance car.. 5.1 is very nice but it still eats paint. high boiling temp and doesnt compress until it boils, less moisture absorbtion, rubber friendly, low compressability.

5 or silicone fluid is good for sunday drivers and trailer queens as you can leave it in the car for a long time and it doesnt eat paint. it is however 4 times more compressible than dot 4 and leads to spongy pedals in non-ABS vehicles. and while it doesnt boil easily, it more compressible as the temp goes up regardless of whethers it boiling or not. also very prone to micro-aeration..

Ill be using 5.1 in mine

http://www.stoptech.com/whitepapers/brakefluid1a.htm

|UPDATED|12/1/2004 1:18:24 AM (AZT)|/UPDATED|

Good info Ben. Just confirms what I would have said but did not have the specs to back up. I did not know about DOT 5.1 Very interesting. Thanks.