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MNBoxster · 04-27-2007, 05:28 PM

Hi,

FYI - DOT 5 is a Silicone based Fluid, while DOT 3 & 4 are Hydraulic Fluids the primary component of which is Polyalkylene Glycol Ether, a synthetic, but often mixed with a mineral oil base. DOT 5.1 is also a Polyalkylene Glycol Ether Fluid, but about half the viscosity of DOT 3, 4, and 5. It was designed specifically for Winter or Cold Climate use, though most European Cars are filled with DOT 5.1 at the Factory. DOT 2 is Castor Oil and used only in Antiques and Collectables because of it's viscosity and friendliness to the Seals, etc. It does not meet the specs of a modern ABS System.

DOT 5 was developed by the US Military which used it since 1982. It has a very high Boiling Point. But, the problem with Silicone is that unlike the Glycol based fluids, it does not mix with water. But, water still invades the system, and since it's specific gravity if higher than that of Silicone, it will migrate to the lowest point in the Brake System, usually the Calipers where it can be compressed into a steam bubble causing Fade, even though the Fluid itself hasn't reached it's Boiling Point.

Also, this water collecting in the Calipers accelerates corrosion and breakdown of the Piston Seals. Despite superior performance, increased maintenance costs from the use of DOT 5 has caused the Military to revert back to Glycol based fluids.

But, Racing, where maintenance costs are not really a factor, continues to use DOT 5 for it's performance. But, unless you are going to swap fluid every 6 mos., DOT 5 should be avoided.

I use ATE Typ 200 (Gold) and Super Blue in my Boxster. But use Castrol LMA (Low Moisture Accumulation) in both my Esprit and 240Z because of the Rubber (as opposed to Butyl) Seals and it is quite good as well as being a couple bucks cheaper.

Personally, I always blow at least a quart through the system to aid in removing contaminates and particulates which come from the breakdown of seals and other parts.

Just be sure to bleed in the proper order. Start with the wheel furthest from the Master Cylinder and work your way backwards to it. Also, be sure to bleed the outboard valve on the Caliper last (the Boxster has 2 Bleed Valves per Caliper). This way, any air which could migrate from the inboard Bleeder to the outer set of pistons is extracted.

As mentioned, the Motive Bleeder works best and is really necessary on an ABS system. Just be sure not to exceed 20PSI to insure that you don't blow the Seals on a used Master Cylinder. This takes a little more time, but is cheap insurance. Good Luck! ...

Happy Motoring!... Jim'99

04-27-2007, 05:28 PM	#6
MNBoxster Registered User Join Date: Sep 2005 Location: Minneapolis/St. Paul, Minnesota, USA Posts: 3,308	Hi, FYI - DOT 5 is a Silicone based Fluid, while DOT 3 & 4 are Hydraulic Fluids the primary component of which is Polyalkylene Glycol Ether, a synthetic, but often mixed with a mineral oil base. DOT 5.1 is also a Polyalkylene Glycol Ether Fluid, but about half the viscosity of DOT 3, 4, and 5. It was designed specifically for Winter or Cold Climate use, though most European Cars are filled with DOT 5.1 at the Factory. DOT 2 is Castor Oil and used only in Antiques and Collectables because of it's viscosity and friendliness to the Seals, etc. It does not meet the specs of a modern ABS System. DOT 5 was developed by the US Military which used it since 1982. It has a very high Boiling Point. But, the problem with Silicone is that unlike the Glycol based fluids, it does not mix with water. But, water still invades the system, and since it's specific gravity if higher than that of Silicone, it will migrate to the lowest point in the Brake System, usually the Calipers where it can be compressed into a steam bubble causing Fade, even though the Fluid itself hasn't reached it's Boiling Point. Also, this water collecting in the Calipers accelerates corrosion and breakdown of the Piston Seals. Despite superior performance, increased maintenance costs from the use of DOT 5 has caused the Military to revert back to Glycol based fluids. But, Racing, where maintenance costs are not really a factor, continues to use DOT 5 for it's performance. But, unless you are going to swap fluid every 6 mos., DOT 5 should be avoided. I use ATE Typ 200 (Gold) and Super Blue in my Boxster. But use Castrol LMA (Low Moisture Accumulation) in both my Esprit and 240Z because of the Rubber (as opposed to Butyl) Seals and it is quite good as well as being a couple bucks cheaper. Personally, I always blow at least a quart through the system to aid in removing contaminates and particulates which come from the breakdown of seals and other parts. Just be sure to bleed in the proper order. Start with the wheel furthest from the Master Cylinder and work your way backwards to it. Also, be sure to bleed the outboard valve on the Caliper last (the Boxster has 2 Bleed Valves per Caliper). This way, any air which could migrate from the inboard Bleeder to the outer set of pistons is extracted. As mentioned, the Motive Bleeder works best and is really necessary on an ABS system. Just be sure not to exceed 20PSI to insure that you don't blow the Seals on a used Master Cylinder. This takes a little more time, but is cheap insurance. Good Luck! ... Happy Motoring!... Jim'99