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From digest.v4.n621 Mon May 20 14:39:49 1996
From: Pete Read <>
Date: Mon, 20 May 1996 17:02:07 -0700
Subject: Re: Brake Fluid Specs, Wet/Dry

Terry Lee asks:
>On Brake Fluid rating, what is meant by dry and wet boiling point?
>And why would you know which one pertains to your application
>at any given time?


Brake fluid is hydroscopic (absorbs water). When fresh from the can, it can be considered "dry" with the higher boiling point. That's why racers and people doing driver's schools change the brake fluid just before events. Over time, brake fluid absorbs water lowering its boiling point to the "wet" level.

For street cars, wet boiling point numbers are more important than dry because the fluid stays in for quite a while (one to two years). After a few months, with exposure to humid air, the brake fluid performance is probably closer to the wet than dry point.

Here's some other brake fluid info I posted a while back...

Brake Fluid Recommendations

  1. Normal Driving - Castrol LMA (Low Moisture Absorption), DOT 4, 446F dry and 311F wet boiling points, about $5 per quart, changed every year or two.
  2. Driver's schools (brakes at very high temp) - ATE Super Blue, DOT 4 spec, 536F dry and 392F wet boiling points, about $11 per quart, changed before every driver's school if it's more than a month old. In my case, about three changes a year for five or six schools.

Brake fluid needs to be changed for two reasons, maintenance and performance (it takes about a quart to flush the system).

Maintenance - changing old brake fluid removes water from the brake system. Brake fluid is hydroscopic, it absorbs water. Old brake fluid must be flushed out or water absorbed by the fluid eventually causes internal rust on the disk calipers and pistons.

Performance - changing old brake fluid helps high temperature operation because fresh (dry) brake fluid has a higher boiling point than older (wet) brake fluid. If brake fluid boils, compressible gas bubbles form, resulting in a very spongy brake pedal.

DOT Brake Fluid Specs

                    DOT 3  DOT 4  DOT 5
                    -----  -----  -----
Dry Boiling Point   401F   446F   500F
Wet Boiling Point 284F 311F 356F

The DOT 3 and DOT 4 specifications are for glycol based (regular) brake fluid, while DOT 5 is for silicone.

Silicone DOT 5 is not compatible with the ABS hydraulic control unit because it doesn't lubricate the ABS pump like glycol based fluid. Silicone is slightly more compressible than glycol fluid. It has a higher wet boiling point because it doesn't absorb moisture like glycol fluid. This lack of moisture absorption causes problems when water gets in the brake system. Instead of mixing with the brake fluid (so it can be flushed out easily), the moisture gets trapped at low points in the system and causes rust.


Pete Read
'88 M5

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