originally published as BRAKE FLUID ••••• which one to use? by Tiger Tom in the December 2000 RootesReview:

One of the failure modes of our hydraulic systems (brake and clutch) is the deterioration of the rubber seals to the point they no longer retain the fluids in the cylinders. We call it leaking. The seals swell, become softer, eventually disintegrate and finally lose the elasticity characteristics essential for proper operation.

It is generally known that we should not use certain brake fluid types in our hydraulic systems. But which ones? Should we use only DOT 4? Or DOT 5? Since we know the major failure mode IS the deterioration of the seal from the swelling process, why not see which fluids cause the most swelling?

During the next several months we will provide a monthly update on the amount of swelling produced by four popular hydraulic brake fluid types in the Rootes Review. You can then judge which fluid to use.

Remember seal deterioration, while the major failure mode, is not the only factor to consider. The hygroscopic (water absorption) effects, boiling points for racing applications, and the tendency to destroy paint are other factors to consider as well.

The Test

This test includes the rubber seals taken from four Girling SP2230 Brake master Cylinder kits. Each seal was placed in a vial of the following brake fluids.

  1. Silicone DOT 5
  2. CastroI GT LMA DOT 3 & 4
  3. Valvoline Synthetic DOT 3 & 4
  4. Prestone DOT 3

The seal outside diameter was measured using a micrometer before being submersed in fluid, two weeks after the initial submersion and then once a month until the test was terminated.

The Results

Measurement PeriodSeal Swelling (inches)
Initial (dry diameter).000 (.783).000 (.785).000 (.779).000 (.781)

Updated by Tiger Tom in the March 2001 RootesReview;


This is the second in a series of monthly tests to quantify the amount of seal swelling that occurs when using various types of brake fluids. Among the various failure mechanisms of hydraulic systems, seal swelling and the resultant deterioration is a major cause of hydraulic system failure. There is no intent in this study to correlate swelling results to MTBF (Mean Time To Failure) of a typical hydraulic system.

Keep in mind that swelling is only one cause of failure, not the only cause. The hygroscopic characteristics of DOT 3 & 4 fluids can cause severe corrosion of cylinders to a degree that they must be sleeved to be repaired. They also destroy paint. Each type of fluid exhibits its own type of problems for a British car owner.

Unfortunately, as British car owners, we do not have the liberty of choosing a fluid that provides the most reliable service and highest performance. Instead, we must choose the best compromise between cost (as in rebuilding), reliability and performance for our particular usage. AHHHH, the thrills of owning a Sunbeam.

SWELL RESULTS (pun intended)

Measurement PeriodSeal Swelling (inch)

Brake seal outside diameter increase measured on a monthly basis after being submersed in the following fluids for the stated times

DOT 3 & 4
DOT 3 & 4
Initial (dry diameter).000 (.783).000 (.785).000 (.799).000 (.781)

Updated again by Tiger Tom in the May 2001 RootesReview:

From Tiger Tom (in which he updates his periodic measurements of o-rings in various kinds of brake fluid):

We have jumped from three months results in the last Rootes Review to six months in this issue. The seals have remained dimensionally stable during the six month period. Another area of seal deterioration is the elastometric characteristic, that is, the firmness of the rubber. I have not been able to accurately quantify this characteristic, due to the small round shape of the seal. I believe there is a relationship between dimension (swell) and hardness…. It is my aim to establish a time-line to failure for “mushiness”, comparing silicone fluid to the others. We will publish when there is a change or on a quarterly basis each year.

Leave a Reply