Bleeding Water from Systems with Silicone Fluid

Because even a tiny amount of water will lead to corrosion, and water in a caliper or wheel cylinder can boil at relatively low temperatures, it is advisable to bleed the system every six to 12 months to eliminate the water.

Hazard to Paint

Unlike glycol fluids, silicone fluids do not damage paint. This is of particular importance in regard to show cars where a spill or leak of glycol fluid can have seriously ugly results. A newly rebuilt and scrupulously clean brake system filled with silicone fluid should outlast a system filled with glycol fluid by several times.

At this point, the silicone fluids have what appears to be an obvious advantage over glycol based fluids. Given all the trouble caused by water contamination of glycol based brake fluid, silicone fluid has some appeal. However there are some drawbacks, and we need to understand those before we start making choices.

Drawbacks of Using Silicone Brake Fluid

Air Solubility

Silicone fluid will absorb more air because there is more “room” between the molecules that make up the fluid. The term “dissolved air” (air absorbed from the atmosphere) should not be confused with the term “entrapped” or “free air” since their effects on brake system performance can be entirely different. Air that has been absorbed from the atmosphere does not result in an increase in fluid or system volume, whereas entrapped air or free air does occupy system volume and can be easily compressed when force is applied to the system.

Compressibility

Because of the dissolved air, silicone fluids are up to three times more compressible than glycol based fluids. This can contribute to a slightly spongy feeling brake pedal, particularly near the higher end of their temperature range but well below the dry boiling point. While this is of absolutely no consequence for normal street use, this is why silicone fluids are not used in race cars. A spongy pedal makes it difficult to modulate the brake pressure under racing conditions.

Bleeding Air From Systems with Silicone Fluid

Silicone fluids have a higher viscosity, which combined with a greater tendency to absorb air leads to slow rates of fill and retention of free air entrapped during filling, which makes bleeding the system difficult. Because air bubbles do not easily dissipate in silicone brake fluid, special care must be used to prevent them from forming during pouring and bleeding operations. The best way to bleed a silicone fluid system is with a professional brake bleeder with a sealed fluid reservoir separated from an air chamber by a rubber diaphragm. The air pressure forces the fluid from the chamber into the reservoir and through the lines to the individual bleed screws. This is very effective, but few individuals will find them practical because of the large amounts of brake fluid they use. There are two vacuum bleeders that will work too. These are the “Professional Vacuum Brake Bleeder” (Moss #386-225), and the “Vacuum Brake Bleeder” kit (Moss # 056-671) will also work. If you do not have either of those, bleed the system with slow even pedal strokes. Avoid “pumping” the pedal, which will introduce air bubbles into the system. It may be necessary to bleed the system again in a day or so if there were any air bubbles that wouldn’t bleed out the first time. Be aware that air in solution in silicone fluid will form bubbles if the atmospheric pressure drops, as it does when you go up quite a bit in elevation. This will result in a spongy pedal that will persist until the air is bled out of the system.