Silicone Brake Fluid

911 and Porsche World

Porsche recommends that the brake fluid be replaced every two years. Porsche also goes on to recommend that we use only polyglycol, DOT 3 or DOT 4 fluids and not DOT 5 silicone fluid. Furthermore they say that we should not add or mix DOT 5 silicone type brake fluid with the brake fluid in your car as severe component corrosion may result. Such corrosion could lead to brake system failure. 

For the model year 1993 and forward the brake systems in all Porsche models were filled with an improved type of brake fluid, ATE type 200. ATE Type 200 fluid was a new improved DOT 4 fluid with a higher dry boiling point, a higher wet boiling point which extended the life of the fluid and resulted in longer change intervals (every 3 years). This same brake fluid is sold on the aftermarket as ATE Blue and is a superior normal DOT 4 brake fluids. 

Brake fluid comes in DOT 3, DOT 4 or DOT 5 ratings. The DOT standards for brake fluids were established in 1972. When the National Highway Traffic Safety Administration, Department of Transportation (NHTSA) set up the requirements for brake fluids they determined that there was a need for two grades of fluid until an all-weather fluid was developed with viscosity and boiling point characteristics suitable for all braking systems. In order to provide added protection against vapor locking and fade in severe braking service, DOT 4 fluid is recommended. But in such applications it is important to note that the same higher viscosity that helps eliminated vapor locking and fade may result in poorer system performance in very cold weather Also, it should be noted that the high boiling points are sacrificed in the DOT 3 fluid for low viscosities for use at low temperatures. These differences between the viscosities of the DOT 3 and DOT 4 fluids are necessary to cover the specified operating temperature ranges and as such make it necessary to maintain both DOT 3 and DOT 4 brake fluids. 

DOT 5 fluid is supposed to be the all-weather fluid that is mentioned in the preceding paragraph and it can be used as a replacement for both DOT 3 and DOT 4 brake fluids. They test all three fluids for a great number of different characteristics, however, the most interesting characteristics to us are the equilibrium reflux boiling point (dry boiling point), the wet equilibrium reflux boiling point (wet boiling point) and the kinematic viscosities (viscosity at cold temperatures). Dry boiling point: These are the minimum boiling temperatures allowed for the various grades of fluid (this test simulates the boiling point when the fluid is new). 

DOT 3 401° F
DOT 4 446° F
DOT 5 500° F 

Wet boiling point: These are the minimum boiling temperatures allowed for the various grades of fluid when wet (this test is a simulation of the boiling point after the absorption of moisture from air) 

DOT 3 284° F
DOT 4 311° F
DOT 5 356° F 

NOTE: These are the minimum requirements and there are brake fluids available that will exceed these minimum requirements. 

Kinematic viscosities: All brake fluids (DOT 3, DOT 4 and DOT 5) must meet a minimum viscosity test of not less than 1.5 centestokes at 100° C (212° F) and must not be more than the following to meet their various classifications (the larger numbers indicate higher kinematic viscosities just like with motor oils). 

DOT 3 1500 Centestokes at minus 40° F
DOT 4 1800 Centestokes at minus 40° F
DOT 5 900 Centestokes at minus 40° F. 

There are some advantages to silicon brake fluid over conventional polyglycol brake fluids. It is permanent, it does not absorb moisture, it does not boil, it helps prevent corrosion rather than causing it, it will not damage paint if it is spilled on the paint and it helps lubricate seals and other components in the brake system. 

The disadvantages, though minor, can still be very annoying. Silicon brake fluids are slightly compressible, or at least they appear to be because of their affinity for air, which results in a different pedal feel from that of a system which uses a conventional non-compressible polyglycol fluid. Because of silicon brake fluid's affinity for air and their surface tension characteristics they tend to cling to any air bubbles which get into the braking system making the system very difficult to bleed properly. 

The brake fluids required for our Porsches by Porsche AG are either DOT 3 or DOT 4 in new and unused condition (that means that the can should be sealed so that there is no moisture in the fluid). Porsche recommends that we changed the brake fluid every two years (three years with the new ATE type 200 brake fluid) because the conventional polyglycol fluids are hygroscopic and will absorb moisture. When the fluid absorbs moisture, two things happen: the boiling temperature goes down and the absorbed moisture is corrosive to the brake components. Minimum wet boiling point is specified for DOT 3 is 284° F and for DOT 4 is 311° F and because these fluids are hygroscopic they commonly will approach these minimum limits. In contrast DOT 5 silicon fluids have a wet boiling temperature of 356° F and because they are not hygroscopic they will never approach the wet boiling limit. 

The silicon brake fluids are DOT 5 and as such are required to be compatible with all conventional DOT 3 and DOT 4 fluids in order to comply with the federal regulations for DOT 5. There are also other DOT 5 brake fluids that while they are synthetic are not silicone based and as a result are more similar to the conventional DOT 3 and DOT 4 brake fluids that we are familiar with. 

Silicon brake fluid is inert and is supposed to be the only universally compatible brake fluid. However silicon is not miscible and will not mix with other types of brake fluids. Although the silicon fluids can be used with conventional polyglycol fluids, they will not mix and the conventional fluid still in the brake system can continue to absorb any moisture in the system components. 

To gain maximum benefits for the silicon brake fluids the complete brake system should be completely disassembled and cleaned before adding the silicon brake fluid. Then because of the silicon brake fluid's affinity for air, it is recommended that a pressure bleeder be used for bleeding the system. 

I have had personal experience with silicon brake fluid in both street and racing cars. And indeed have experienced difficulties getting the systems properly bled. Even when we were sure we had properly bled the brake system, the brake pedal always had a strange, soft feel to it. We attempted to use silicon brake fluid in a Porsche 935 race car at Daytona in 1981 for the twenty four hour race because of the problems we have in Florida with the high humidity lowering the boiling point of the brake fluid. It probably would have achieved our objective; however, the drivers didn't like the pedal feel and requested that we change it back to conventional fluid. The spongy pedal thing gets worse with heat and in our experience with it at Daytona the driver, Bobby Rahal, told us if we didn't take it out we would have to drive the car ourselves; it was scaring him to death. At that time there was no chicane on the back strait away and they would come into the braking area for Turn one at about 200 mph before braking. At that point they were committed to turn one, which was a little like threading a needle and the soft brake pedal was very unsettling to say the least. Rahal said that the pedal would get worse each lap as the brakes got warmer. We returned to conventional brake fluid and won that years 24-hour race. 

In street cars the silicon fluid has advantages which may outweigh its disadvantages, particularly for older cars that do not get driven very much. Moisture absorbed by conventional polyglycol brake fluid cause serious corrosion of metal parts and deterioration of rubber seals and hoses in the brake system. However, in order to take advantage of that attribute the brake system must be completely free of any polyglycol brake fluid. If any polyglycol brake fluid remains in the system any moisture in the system will be attracted to those small pockets of polyglycol brake fluid causing corrosion