refer to the threads on DOT 5 and 5.1 before going down that rabbit hole.
I did ....you?
I guess "hot shot" says it all. I didn't pee in your Cheerios or slam you....I suggested that he make up his own mind based on readily available comments both pro and con.... So, unless you have 50 years at the track as a rider, driver and owner plus a chemical engineering degree and own a company that does research under contract to automotive and poly film manufacturer's, I may have a leg up on you about this topic. Let's just leave it that not one single race track...bike or car, allows DOT5 to be used in sanctioned racing....and that's what the fluid was originally developed for. Harley was the only manufacturer that tried to use it....and stopped in 2005 after 7 years of specifying its use. No car manufacturer has ever specified DOT 5 for use because of the ABS tendency to foam it and lose all braking. The European M/C mfr's who spec 5....are specifying 5.1 because it can be used with their ABS systems. It's glycol + borates....just like 3 and 4. Borates raise the boiling point.
The US military specifies DOT 5 due to the long term storage of the vehicle pool....however, if you know someone who is in the motor pool....give them a call and be prepared to listen to how much hatred there is from drivers over the use of DOT 5. There are reports of having to use paint filters when adding to the master due to 'gummy bears' in the cans.
A "+1" doesn't really get to the point, in my opinion. DOT 5 is easier on paint at three to four times the cost of the alternatives. DOT 3, 4 and 5.1 have 3-4 yr. life cycles, so you would be messing with the fluid, max, three times over a ten year period....if diligent about it. 5...maybe twice.
As with any product there are several trade-offs to consider:
Silicone fluids are not miscible with water. It’s the classic oil and vinegar scenario. DOT 5 Silicone, being less dense, tends to float on water. Therefore, if water ever entered the system, ...which it does through osmosis of the hydraulics's soft materials and most comes from the vent in the master cylinder cap and resultant condensation in the air space above the fluid., would collect at the lowest point, meaning the bottom of the caliper. Water in the bottom of the caliper could be hard to get out without removing the caliper and rotating the caliper to place the bleed nipple at the lowest point. Not many who use 5 are aware they have to do this.....so they don't. Without non-corrosive materials, such as Phenolic pistons etc. there is a higher risk of corrosion welding than if using straight glycol based products. They suspend the moisture in their entire volume until that absorption capacity is exceeded. If you are going to have water in your brake fluid, you actually want it dispersed throughout the fluid because it minimizes the chance of corrosion caused by localized pockets of water. It also prevents a pocket of water in a caliper boiling, which would occur around 212ºF, much lower than the boiling point of the brake fluid. A heavy braking situation can cause rotor to pad contact heat of 1000F.
In addition, the stopping ability of a system can be compromised since Silicone based brake fluids do compress...especially at higher operating temperatures. I wouldn't really consider that much of a problem on a street bike.....unless doing a lot of frequent heavy braking. DOT 5 is more compressible because it will absorb more air than a glycol based fluid. The air is in solution, and should not be confused with air bubbles. The dissolved air gives a slightly spongy pedal feel, which most people cannot detect. It makes silicone fluid a poor choice for racing, but it is considered an appropriate choice for a classic that is not driven daily. Bleeding systems with silicone takes more time and may have to be repeated because it takes time for the air to works its way out.
